Allow C++ custom class to define __repr__ and use it from Python (#100724)
When handling custom classes from Python, it is nice to be able to specify how they are displayed to the user.
Out of the two standard functions to do this, only `__str__` could be implemented in C++. This PR add `__repr__` to the allowlist of magic methods.
The second commit tweaks the default output of `__str__` to make it more informative, but I can remove the change if you want.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100724
Approved by: https://github.com/ezyang
