`PyTorchFileWriter` should drop the GIL while writing files
Labels: module: serialization, triaged

### 🐛 Describe the bug

`torch.save` does all of its actual file IO via the internal `PyTorchFileWriter` class.

However, this class does not drop the GIL while doing file I/O, resulting in long hangs if one thread is executing a `torch.save`. We can see this by repeatedly saving a tensor on a thread while watching for overly-long hangs on another:

https://gist.github.com/nelhage/1567e34c9e385c7e57ce88440a3b1525

Running this, I see output like

```
❯ python torch_save.py
Unexpected latency! Sleep=0.82s
Saved tensor in 0.80s
Unexpected latency! Sleep=0.19s
Saved tensor in 1.00s
Unexpected latency! Sleep=0.15s
Unexpected latency! Sleep=0.17s
Saved tensor in 1.06s
Unexpected latency! Sleep=0.19s
Saved tensor in 1.05s
Unexpected latency! Sleep=0.81s
Saved tensor in 1.03s
```

By comparison, if I save the tensor using `numpy`, we see no such hangs:
```
❯ python torch_save.py  numpy
Saved tensor in 0.54s
Saved tensor in 0.63s
Saved tensor in 0.65s
Saved tensor in 0.65s
Saved tensor in 0.74s
Saved tensor in 0.65s
Saved tensor in 0.65s
Saved tensor in 0.72s
…
```



### Versions

```
PyTorch version: 1.13.0+cu117
Is debug build: False
CUDA used to build PyTorch: 11.7
ROCM used to build PyTorch: N/A

OS: Ubuntu 22.04.1 LTS (x86_64)
GCC version: (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
Clang version: 16.0.0-++20220813052912+eaf0aa1f1fbd-1~exp1~20220813173018.344
CMake version: version 3.22.1
Libc version: glibc-2.35

Python version: 3.11.0 (main, Nov  6 2022, 16:51:40) [GCC 11.3.0] (64-bit runtime)
Python platform: Linux-5.15.0-56-generic-x86_64-with-glibc2.35
Is CUDA available: True
CUDA runtime version: Could not collect
CUDA_MODULE_LOADING set to: LAZY
GPU models and configuration: GPU 0: NVIDIA GeForce RTX 2080 SUPER
Nvidia driver version: 515.86.01
cuDNN version: Probably one of the following:
/usr/lib/x86_64-linux-gnu/libcudnn.so.8.7.0
/usr/lib/x86_64-linux-gnu/libcudnn_adv_infer.so.8.7.0
/usr/lib/x86_64-linux-gnu/libcudnn_adv_train.so.8.7.0
/usr/lib/x86_64-linux-gnu/libcudnn_cnn_infer.so.8.7.0
/usr/lib/x86_64-linux-gnu/libcudnn_cnn_train.so.8.7.0
/usr/lib/x86_64-linux-gnu/libcudnn_ops_infer.so.8.7.0
/usr/lib/x86_64-linux-gnu/libcudnn_ops_train.so.8.7.0
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
Address sizes:                   43 bits physical, 48 bits virtual
Byte Order:                      Little Endian
CPU(s):                          24
On-line CPU(s) list:             0-23
Vendor ID:                       AuthenticAMD
Model name:                      AMD Ryzen 9 3900X 12-Core Processor
CPU family:                      23
Model:                           113
Thread(s) per core:              2
Core(s) per socket:              12
Socket(s):                       1
Stepping:                        0
Frequency boost:                 enabled
CPU max MHz:                     4672.0698
CPU min MHz:                     2200.0000
BogoMIPS:                        7585.70
Flags:                           fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf rapl pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb cat_l3 cdp_l3 hw_pstate ssbd mba ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 cqm rdt_a rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local clzero irperf xsaveerptr rdpru wbnoinvd arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif v_spec_ctrl umip rdpid overflow_recov succor smca sme sev sev_es
Virtualization:                  AMD-V
L1d cache:                       384 KiB (12 instances)
L1i cache:                       384 KiB (12 instances)
L2 cache:                        6 MiB (12 instances)
L3 cache:                        64 MiB (4 instances)
NUMA node(s):                    1
NUMA node0 CPU(s):               0-23
Vulnerability Itlb multihit:     Not affected
Vulnerability L1tf:              Not affected
Vulnerability Mds:               Not affected
Vulnerability Meltdown:          Not affected
Vulnerability Mmio stale data:   Not affected
Vulnerability Retbleed:          Mitigation; untrained return thunk; SMT enabled with STIBP protection
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; Retpolines, IBPB conditional, STIBP always-on, RSB filling, PBRSB-eIBRS Not affected
Vulnerability Srbds:             Not affected
Vulnerability Tsx async abort:   Not affected

Versions of relevant libraries:
[pip3] mypy-extensions==0.4.3
[pip3] numpy==1.23.4
[pip3] torch==1.13.0
[conda] Could not collect
```

cc @mruberry