Changes warnings generated in cpp to show point of Python origination (#36052)
Summary:
Today in PyTorch, warnings triggered in C++ are printed to Python users like this:
`../aten/src/ATen/native/BinaryOps.cpp:81: UserWarning: Integer division of tensors using div or / is deprecated, and in a future release div will perform true division as in Python 3. Use true_divide or floor_divide (// in Python) instead.`
This may be unhelpful to Python users, who have complained it's difficult to relate these messages back to their programs. After this PR, warnings that go through the PyWarningHandler and allow it to add context print like this:
```
test/test_torch.py:16463: UserWarning: Integer division of tensors using div or / is deprecated, and in a future release div will perform true division as in Python 3. Use true_divide or floor_divide (// in Python) instead. (Triggered internally at ../aten/src/ATen/native/BinaryOps.cpp:81.)
cpu_result = getattr(cpu_tensor, op_str)(*cpu_args)
```
This relates the warning back to the user's program. The information about the cpp file and line number is preserved in the body of the warning message.
Some warnings, like those generated in the JIT, already account for a user's Python context, and so they specify that they should be printed verbatim and are unaffected by this change. Warnings originating in Python and warnings that go through c10's warning handler, which prints to cerr, are also unaffected.
A test is added to test_torch.py for this behavior. The test relies on uint8 indexing being deprecated and its warning originating from its current header file, which is an unfortunate dependency. We could implement a `torch.warn` function, instead.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36052
Differential Revision: D20887740
Pulled By: mruberry
fbshipit-source-id: d3515c6658a387acb7fccaf83f23dbb452f02847
