FSDP crashes when submodule calls method that isn't `forward()`
Labels: triaged, module: fsdp

### 🐛 Describe the bug

I am getting various runtime errors given an FSDP module that wraps multiple children modules, where in the forward pass, we invoke a submodule's non-forward method. The autowrap policy wraps each submodule separately. The minimal example below should make this more clear:

Run with (at least 2 GPUs): `torchrun --standalone --nnodes 1 --nproc-per-node 2 <script.py>

```python
from functools import partial

import torch
import torch.distributed as dist
import torch.nn as nn
from torch.distributed.fsdp import FullyShardedDataParallel as FSDP, ShardingStrategy
from torch.distributed.fsdp.wrap import _module_wrap_policy

# This code is a "dummy" VisionTransformer that just implements the offending partial logic from a default pretrained ViT
# Our actual code uses a model loaded from `timm` (PyTorch Image Models); full Gist is linked below.

class VisionTransformer(nn.Module):
    def __init__(self) -> None:
        super().__init__()
        self.embed_dim = 1024

        # Mimics the "Patch Embedding" for a ViT-Large w/ Patch Size = 14
        self.patch_proj = nn.Conv2d(in_channels=3, out_channels=self.embed_dim, kernel_size=14, stride=14, bias=True)

    def forward_features(self, imgs: torch.Tensor) -> torch.Tensor:
        patches = self.patch_proj(imgs)                                               # [bsz, embed_dim, 16 = (224 / 14), 16 = (224 / 14)]
        patch_embeddings = patches.flatten(2).transpose(1, 2)       # [bsz, 256 = 16 * 16, 1024]
        return patch_embeddings

    def forward(self, imgs: torch.Tensor) -> torch.Tensor:
        return self.forward_features(imgs).sum(dim=1)

class LinearProjector(nn.Module):
    def __init__(self, in_dim: int, out_dim: int) -> None:
        super().__init__()
        self.projector = nn.Linear(in_dim, out_dim)

    def forward(self, patch_embeddings: torch.Tensor) -> torch.Tensor:
        return self.projector(patch_embeddings)

# === Actual Network ===
class Net(nn.Module):
    def __init__(self) -> None:
        super().__init__()
        self.vit, self.projector = VisionTransformer(), LinearProjector(in_dim=1024, out_dim=256)

    def forward(self, imgs: torch.Tensor) -> torch.Tensor:
        patch_embeddings = self.vit.forward_features(imgs)  # [ERRORS HERE]
        return self.projector(patch_embeddings)

# === Main ===
def bug_fsdp() -> None:
    dist.init_process_group(backend="nccl", init_method="env://")
    torch.cuda.set_device(dist.get_rank() % torch.cuda.device_count())

    # Initialize Network
    net = Net()
    
    # FSDP w/ custom module-based autowrap policy
    auto_wrap_policy = partial(_module_wrap_policy, module_classes={VisionTransformer, LinearProjector})
    net = FSDP(
        net,
        auto_wrap_policy=auto_wrap_policy,
        sharding_strategy=ShardingStrategy.FULL_SHARD,
        device_id=torch.cuda.current_device(),
        limit_all_gathers=True,
    )

   # Run a forward pass w/ dummy input (bsz = 4)
   dummy_input = torch.randn(4, 3, 224, 224)
   net(dummy_input)    # CRASH!

if __name__ == "__main__":
     bug_fsdp()
```

This results in the following error message:

```
Traceback (most recent call last):                                                                                                                                                                                    
  File "/mnt/fsx/skaramcheti/code/prismatic-vlms/x-reference/bugs-fixes/bug_fsdp.py", line 74, in <module>                                                                                                            
    bug_fsdp()                                                                                                                                                                                                        
  File "/mnt/fsx/skaramcheti/code/prismatic-vlms/x-reference/bugs-fixes/bug_fsdp.py", line 70, in bug_fsdp                                                                                                            
    net(dummy_input)  # CRASH!                                                                                                                                                                                        
  File "/home/ubuntu/mambaforge/envs/fsdp-debug/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1501, in _call_impl                                                                                    
    return forward_call(*args, **kwargs)                                                                                                                                                                              
  File "/home/ubuntu/mambaforge/envs/fsdp-debug/lib/python3.10/site-packages/torch/distributed/fsdp/fully_sharded_data_parallel.py", line 748, in forward                                                             
    output = self._fsdp_wrapped_module(*args, **kwargs)                                                                                                                                                              
  File "/home/ubuntu/mambaforge/envs/fsdp-debug/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1501, in _call_impl                                                                                    
    return forward_call(*args, **kwargs)                                                                                                                                                                              
  File "/mnt/fsx/skaramcheti/code/prismatic-vlms/x-reference/bugs-fixes/bug_fsdp.py", line 46, in forward                                                                                                             
    patch_embeddings = self.vit.forward_features(imgs)  # [ERRORS HERE]                                                                                                                                               
  File "/mnt/fsx/skaramcheti/code/prismatic-vlms/x-reference/bugs-fixes/bug_fsdp.py", line 22, in forward_features                                                                                                   
    patches = self.patch_proj(imgs)  # [bsz, embed_dim, 16 = (224 / 14), 16 = (224 / 14)]                                                                                                                            
  File "/home/ubuntu/mambaforge/envs/fsdp-debug/lib/python3.10/site-packages/torch/nn/modules/module.py", line 1501, in _call_impl                                                                                   
    return forward_call(*args, **kwargs)                                                                                                                                                                             
  File "/home/ubuntu/mambaforge/envs/fsdp-debug/lib/python3.10/site-packages/torch/nn/modules/conv.py", line 463, in forward                                                                                          
    return self._conv_forward(input, self.weight, self.bias)                                                                                                                                                         
  File "/home/ubuntu/mambaforge/envs/fsdp-debug/lib/python3.10/site-packages/torch/nn/modules/conv.py", line 459, in _conv_forward                                                                                    
    return F.conv2d(input, weight, bias, self.stride,                                                                                                                                                                 
RuntimeError: Output 0 of ViewBackward0 is a view and its base or another view of its base has been modified inplace. This view is the output of a function that returns multiple views. Such functions do not allow the output views to be modified inplace. You should replace the inplace operation by an out-of-place one.
```

---

Further context: I'm working on a project where we take the patch features from a (frozen) Vision Transformer backbone and transform them into a different latent space where they're used to decode other modalities (e.g., depth). 

This gist provides an annotated example that reflects our setup a bit better: https://gist.github.com/siddk/db3e8808bed2a9cb90ae62b5338de68d

---

**Some other things I tried (to help speed along debugging) -- all of this is in the linked Gist**:
    - **Setting `use_orig_params=True` results in a different error at the same Conv2D call (`RuntimeError: weight should have at least three dimensions`)
    -  **Freezing the ViT** (as required in our original setup) results in yet another error at the Conv2D call (`RuntimeError: GET was unable to find an engine to execute this computation`)

Interestingly, if we monkey patch the `vit` instance such that `vit.forward = vit.forward_features` and call `self.vit(imgs)` in `Net.forward()` -- **all of these bugs disappear!** 


### Versions

```
PyTorch version: 2.0.1
Is debug build: False
CUDA used to build PyTorch: 11.8
ROCM used to build PyTorch: N/A

OS: Ubuntu 20.04.6 LTS (x86_64)
GCC version: (Ubuntu 9.4.0-1ubuntu1~20.04.2) 9.4.0
Clang version: Could not collect
CMake version: version 3.16.3
Libc version: glibc-2.31

Python version: 3.10.12 | packaged by conda-forge | (main, Jun 23 2023, 22:40:32) [GCC 12.3.0] (64-bit runtime)
Python platform: Linux-5.15.0-1033-aws-x86_64-with-glibc2.31
Is CUDA available: True
CUDA runtime version: 11.8.89
CUDA_MODULE_LOADING set to: LAZY
GPU models and configuration:
GPU 0: NVIDIA A100-SXM4-80GB
GPU 1: NVIDIA A100-SXM4-80GB
GPU 2: NVIDIA A100-SXM4-80GB
GPU 3: NVIDIA A100-SXM4-80GB
GPU 4: NVIDIA A100-SXM4-80GB
GPU 5: NVIDIA A100-SXM4-80GB
GPU 6: NVIDIA A100-SXM4-80GB
GPU 7: NVIDIA A100-SXM4-80GB

Nvidia driver version: 525.85.12
cuDNN version: Could not collect
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

CPU:
Architecture:                    x86_64
CPU op-mode(s):                  32-bit, 64-bit
Byte Order:                      Little Endian
Address sizes:                   46 bits physical, 48 bits virtual
CPU(s):                          96
On-line CPU(s) list:             0-95
Thread(s) per core:              2
Core(s) per socket:              24
Socket(s):                       2
NUMA node(s):                    2
Vendor ID:                       GenuineIntel
CPU family:                      6
Model:                           85
Model name:                      Intel(R) Xeon(R) Platinum 8275CL CPU @ 3.00GHz
Stepping:                        7
CPU MHz:                         2999.998
BogoMIPS:                        5999.99
Hypervisor vendor:               KVM
Virtualization type:             full
L1d cache:                       1.5 MiB
L1i cache:                       1.5 MiB
L2 cache:                        48 MiB
L3 cache:                        71.5 MiB
NUMA node0 CPU(s):               0-23,48-71
NUMA node1 CPU(s):               24-47,72-95
Vulnerability Itlb multihit:     KVM: Mitigation: VMX unsupported
Vulnerability L1tf:              Mitigation; PTE Inversion
Vulnerability Mds:               Vulnerable: Clear CPU buffers attempted, no microcode; SMT Host state unknown
Vulnerability Meltdown:          Mitigation; PTI
Vulnerability Mmio stale data:   Vulnerable: Clear CPU buffers attempted, no microcode; SMT Host state unknown
Vulnerability Retbleed:          Vulnerable
Vulnerability Spec store bypass: Vulnerable
Vulnerability Spectre v1:        Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2:        Mitigation; Retpolines, STIBP disabled, RSB filling, PBRSB-eIBRS Not affected
Vulnerability Srbds:             Not affected
Vulnerability Tsx async abort:   Not affected
Flags:                           fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq monitor ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch invpcid_single pti fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid mpx avx512f avx512dq rdseed adx smap clflushopt clwb avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves ida arat pku ospke

Versions of relevant libraries:
[pip3] numpy==1.25.2
[pip3] torch==2.0.1
[pip3] torchaudio==2.0.2
[pip3] torchvision==0.15.2
[pip3] triton==2.0.0
[conda] blas                      2.116                       mkl    conda-forge
[conda] blas-devel                3.9.0            16_linux64_mkl    conda-forge
[conda] ffmpeg                    4.3                  hf484d3e_0    pytorch
[conda] libblas                   3.9.0            16_linux64_mkl    conda-forge
[conda] libcblas                  3.9.0            16_linux64_mkl    conda-forge
[conda] liblapack                 3.9.0            16_linux64_mkl    conda-forge
[conda] liblapacke                3.9.0            16_linux64_mkl    conda-forge
[conda] mkl                       2022.1.0           h84fe81f_915    conda-forge
[conda] mkl-devel                 2022.1.0           ha770c72_916    conda-forge
[conda] mkl-include               2022.1.0           h84fe81f_915    conda-forge
[conda] numpy                     1.25.2          py310ha4c1d20_0    conda-forge
[conda] pytorch                   2.0.1           py3.10_cuda11.8_cudnn8.7.0_0    pytorch
[conda] pytorch-cuda              11.8                 h7e8668a_5    pytorch
[conda] pytorch-mutex             1.0                        cuda    pytorch
[conda] torchaudio                2.0.2               py310_cu118    pytorch
[conda] torchtriton               2.0.0                     py310    pytorch
[conda] torchvision               0.15.2              py310_cu118    pytorch
```

cc @zhaojuanmao @mrshenli @rohan-varma @awgu @fegin @penguinwu