Eagerly populate python_error::what() when TORCH_SHOW_CPP_STACKTRACES=1 (#65376)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65376
Let's suppose there's a bug in PyTorch and python_error gets thrown
and never gets caught. Typically, you'll get a very useless error
message like this:
```
terminate called after throwing an instance of 'python_error'
what():
Aborted (core dumped)
```
Now, you'll get:
```
what(): unknown Python error (for more information, try rerunning with TORCH_SHOW_CPP_STACKTRACES=1)
```
and with TORCH_SHOW_CPP_STACKTRACES=1 you'll get:
```
what(): error message from Python object
```
If we're OK with making Python exceptions go even slower, we could
eagerly populate unconditionally. I'm also not so happy we don't get
a Python backtrace or the Python error name, that's worth improving
(this is a minimal diff to get the discussion going.)
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D31067632
Pulled By: ezyang
fbshipit-source-id: 9cfda47cafb349ee3d6853cdfb0f319073b87bff
