Avoid temporary buffers for tensors with torch.save. (#80404)
Fix torch.save _open_zipfile_writer optimization that uses a c++ stream when `f` is a os.PathLike.
This fastpath requires that we don't `open()` in python if possible, so don't do it unconditionally.
Fix PyTorchStreamWriter construction binding that takes a buffer object.
Use py::memoryview instead of py::bytes as the former doesn't copy the data.
Validated with a trivial benchmark that calls torch.save in a loop 20x with a 10M elements float32 tensor
either on cpu or cuda. Saved to /dev/null.
Tried two variants 'str' and 'open'
In 'str' we pass the string "/dev/null" to torch.save.
In 'open' we pass `open("/dev/null", "wb")` to torch.save.
Timing in seconds.
Before this patch:
str-cpu :: 0.757
open-cpu :: 0.757
str-cuda :: 1.367
open-cuda :: 1.366
After this patch:
str-cpu :: 0.256
open-cpu :: 0.251
str-cuda :: 0.896
open-cuda :: 0.834
Fixes #ISSUE_NUMBER
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80404
Approved by: https://github.com/jamesr66a
