Rename distributed autograd test to avoid "test" prefix (#27161)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27161
If the base class starts with "Test", pytest would detect it as a
test class to run. However, the base class does not have a proper
`setUp` method to launch processes. As a result, when I execute
the following command, I run into test failures:
py.test test_dist_autograd_fork.py -k test -vs
outputs:
```
test/test_dist_autograd_fork.py::TestDistAutograd::test_autograd_context FAILED
test/test_dist_autograd_fork.py::TestDistAutograd::test_autograd_send_function FAILED
test/test_dist_autograd_fork.py::TestDistAutograd::test_rpc_complex_args FAILED
test/test_dist_autograd_fork.py::DistAutogradTestWithFork::test_autograd_context PASSED
test/test_dist_autograd_fork.py::DistAutogradTestWithFork::test_autograd_send_function PASSED
test/test_dist_autograd_fork.py::DistAutogradTestWithFork::test_rpc_complex_args PASSED
```
Test Plan: Imported from OSS
Differential Revision: D17694165
Pulled By: mrshenli
fbshipit-source-id: 0b8fcb99c76b5139b765831079f083c3122f618a
