nn.TransformerEncoderLayer fastpath (BetterTransformer) is much slower with src_key_padding_mask
Labels: oncall: transformer/mha

### 🐛 Describe the bug

TransformerEncoder runs much slower with src_key_padding_mask than without any padding. On v100, it takes ~8.8ms for bert-base batch size 1 seq 128 with mask set while only takes ~4.5ms without mask.

```
import torch
import timeit

def test_transformerencoder_fastpath():
    """
    Test TransformerEncoder fastpath output matches slowpath output
    """
    torch.manual_seed(1234)
    nhead = 12
    d_model = 768
    dim_feedforward = 4 * d_model
    batch_first = True
    device = "cuda"

    model = torch.nn.TransformerEncoder(
        torch.nn.TransformerEncoderLayer(
            d_model=d_model,
            nhead=nhead,
            dim_feedforward=dim_feedforward,
            batch_first=batch_first),
        num_layers=12,
    ).to(device).half().eval()

    # each input is (input, mask)
    input_value = torch.rand(8, 128, d_model)
    mask_value = [ [0] * 128] + [[0] * 64 + [1] * 64] * 7

    input = torch.tensor(input_value, device=device, dtype=torch.get_default_dtype()).half()  # half input
    src_key_padding_mask = torch.tensor(mask_value, device=device, dtype=torch.bool)  # bool mask

    with torch.no_grad():
        print(f'''With mask: {timeit.timeit("model(input, src_key_padding_mask=src_key_padding_mask)", globals=locals(), number=1000)}"''')
        print(f'''Without mask: {timeit.timeit("model(input)", globals=locals(), number=1000)}''')

test_transformerencoder_fastpath()
```

### Versions

ollecting environment information...
PyTorch version: 1.13.1+cu116
Is debug build: False
CUDA used to build PyTorch: 11.6
ROCM used to build PyTorch: N/A

OS: Ubuntu 20.04.5 LTS (x86_64)
GCC version: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
Clang version: Could not collect
CMake version: version 3.25.0
Libc version: glibc-2.31

Python version: 3.8.10 (default, Nov 14 2022, 12:59:47)  [GCC 9.4.0] (64-bit runtime)
Python platform: Linux-5.15.0-1031-azure-x86_64-with-glibc2.29
Is CUDA available: True
CUDA runtime version: 11.6.124
CUDA_MODULE_LOADING set to: LAZY
GPU models and configuration: GPU 0: Tesla V100-PCIE-16GB
Nvidia driver version: 510.108.03
cuDNN version: Probably one of the following:
/usr/lib/x86_64-linux-gnu/libcudnn.so.8.4.1
/usr/lib/x86_64-linux-gnu/libcudnn_adv_infer.so.8.4.1
/usr/lib/x86_64-linux-gnu/libcudnn_adv_train.so.8.4.1
/usr/lib/x86_64-linux-gnu/libcudnn_cnn_infer.so.8.4.1
/usr/lib/x86_64-linux-gnu/libcudnn_cnn_train.so.8.4.1
/usr/lib/x86_64-linux-gnu/libcudnn_ops_infer.so.8.4.1
/usr/lib/x86_64-linux-gnu/libcudnn_ops_train.so.8.4.1
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):                          6
On-line CPU(s) list:             0-5
Thread(s) per core:              1
Core(s) per socket:              6
Socket(s):                       1
NUMA node(s):                    1
Vendor ID:                       GenuineIntel
CPU family:                      6
Model:                           79
Model name:                      Intel(R) Xeon(R) CPU E5-2690 v4 @ 2.60GHz
Stepping:                        1
CPU MHz:                         2593.993
BogoMIPS:                        5187.98
Hypervisor vendor:               Microsoft
Virtualization type:             full
L1d cache:                       192 KiB
L1i cache:                       192 KiB
L2 cache:                        1.5 MiB
L3 cache:                        35 MiB
NUMA node0 CPU(s):               0-5
Vulnerability Itlb multihit:     KVM: Mitigation: VMX unsupported
Vulnerability L1tf:              Mitigation; PTE Inversion
Vulnerability Mds:               Mitigation; Clear CPU buffers; 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:          Not affected
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:   Mitigation; Clear CPU buffers; SMT Host state unknown
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 rep_good nopl xtopology cpuid pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 movbe popcnt aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch invpcid_single pti fsgsbase bmi1 hle avx2 smep bmi2 erms invpcid rtm rdseed adx smap xsaveopt md_clear

Versions of relevant libraries:
[pip3] numpy==1.24.1
[pip3] torch==1.13.1+cu116
[conda] Could not collect

cc @jbschlosser @bhosmer @cpuhrsch @erichan1