Make context decorator frames identify the context manager (#186502)

Python tracebacks use a frame's code object name, not the wrapper
function's __name__ metadata. torch.utils._contextlib.context_decorator
used nested wrappers named decorate_context and generator_context for every
context-manager decorator, so tracebacks could not distinguish no_grad,
inference_mode, and other context managers.

Rename the generated wrapper code object at decoration time using the
context manager name. This keeps functools.wraps metadata for the decorated
function intact while making traceback frames report the context manager that
is actually active. Generator wrappers use the same path.

Fixes #118747
Generated by my agent

Test Plan:
python test/test_utils.py TestTraceback.test_context_decorator_traceback_frame_name TestTraceback.test_context_decorator_generator_traceback_frame_name
python test/test_utils.py TestTraceback
python test/test_autograd.py TestAutograd.test_no_grad TestAutograd.test_set_grad_generator_functions TestAutograd.test_set_grad_generator_functions_recursive
lintrunner -a
Pull Request resolved: https://github.com/pytorch/pytorch/pull/186502
Approved by: https://github.com/karthickai