Repository: pytorch
Issue: torch.overrides.get_overridable_functions list functions that are not overridable via __torch_function__
Labels: triaged,module: __torch_function__

### 🐛 Describe the bug

In some scenarios, user may add custom functions/methods for torch.Tensor/torch.XXX (such as this official API torch.utils.generate_methods_for_privateuse1_backend #121723 ), torch.overrides.get_overridable_functions may list them as overridable, but they can not.
```python
>>> import torch
>>> def xx():
...   pass
... 
>>> torch.Tensor.xx = xx
>>> xx in set(torch.overrides.get_overridable_functions()[torch.Tensor])
True
```
I'm not very sure its a design or a bug, but may `tested_namespaces` should be defined outside the function?
https://github.com/pytorch/pytorch/blob/3eb84b6343f47bf9ab4fe46079933910f66271be/torch/overrides.py#L1704


### Versions

Collecting environment information...                                      
PyTorch version: 2.4.0a0+git4ba51bb
Is debug build: True                                                       
CUDA used to build PyTorch: None
ROCM used to build PyTorch: N/A
                                                                                                                                                                                                                                                                                                                    
OS: Ubuntu 20.04.1 LTS (x86_64)
GCC version: (Ubuntu 9.4.0-1ubuntu1~20.04.2) 9.4.0                                                                                                                                                                                                                                                                  
Clang version: 10.0.0-4ubuntu1 
CMake version: version 3.27.7                                                                                                                                                                                                                                                                                       
Libc version: glibc-2.31                                                                                                                                                                                                                                                                                            

Python version: 3.10.8 (main, Dec 13 2023, 17:01:57) [GCC 9.4.0] (64-bit runtime)
Python platform: Linux-5.4.0-42-generic-x86_64-with-glibc2.31
Is CUDA available: False
CUDA runtime version: No CUDA
CUDA_MODULE_LOADING set to: N/A
GPU models and configuration: No CUDA
Nvidia driver version: No CUDA
cuDNN version: No CUDA
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):                          104
On-line CPU(s) list:             0-103
Thread(s) per core:              2
Core(s) per socket:              26
Socket(s):                       2
NUMA node(s):                    2
Vendor ID:                       GenuineIntel
CPU family:                      6
Model:                           85
Model name:                      Intel(R) Xeon(R) Gold 6230R CPU @ 2.10GHz
Stepping:                        7
CPU MHz:                         999.998
CPU max MHz:                     4000.0000
CPU min MHz:                     1000.0000
BogoMIPS:                        4200.00
Virtualization:                  VT-x
L1d cache:                       1.6 MiB
L1i cache:                       1.6 MiB
L2 cache:                        52 MiB
L3 cache:                        71.5 MiB
NUMA node0 CPU(s):               0-25,52-77
NUMA node1 CPU(s):               26-51,78-103
Vulnerability Itlb multihit:     KVM: Mitigation: Split huge pages
Vulnerability L1tf:              Not affected
Vulnerability Mds:               Not affected
Vulnerability Meltdown:          Not affected
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl and seccomp
Vulnerability Spectre v1:        Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2:        Mitigation; Enhanced IBRS, IBPB conditional, RSB filling
Vulnerability Srbds:             Not affected
Vulnerability Tsx async abort:   Mitigation; TSX disabled
Flags:                           fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 invpcid_single intel_ppin ssbd mba ibrs ibpb stibp ibrs_enhanced tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid cqm mpx rdt_a avx512f avx512dq rdseed adx smap clflushopt clwb intel_pt avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts hwp hwp_act_window hwp_epp hwp_pkg_req pku ospke avx512_vnni md_clear flush_l1d arch_capabilities

Versions of relevant libraries:
[pip3] flake8==6.1.0
[pip3] flake8-bugbear==23.3.23
[pip3] flake8-comprehensions==3.12.0
[pip3] flake8-executable==2.1.3
[pip3] flake8-logging-format==0.9.0
[pip3] flake8-pyi==23.3.1
[pip3] flake8-simplify==0.19.3
[pip3] mypy==1.8.0
[pip3] mypy-extensions==1.0.0
[pip3] numpy==1.26.0
[pip3] optree==0.10.0
[pip3] torch==2.4.0a0+git4ba51bb
[pip3] torch-mlu==1.19.0+torch2.3.0
[pip3] torch-tb-profiler==0.4.0
[pip3] torchvision==0.18.0a0+2bababf
[conda] Could not collect


cc @hameerabbasi @rgommers @peterbell10 @ezyang