Remove rpc fork and dist autograd fork tests from PyTorch repo (pytorch#29827)

Summary:
Pull Request resolved: pytorch#29827

There are known issues for "fork tests + OMP" in Pytorch, rpc and dist autograd tests use OMP thread pools, this caused rpc fork and dist autograd fork tests to be flaky. So remove these fork tests from PyTorch repo. rpc spawn and dist autograd spawn tests are still running.

Test Plan: unit tests

Differential Revision: D18507384

fbshipit-source-id: 9e239f13850832b4b84724828537f73512f3fca9

Author: zhaojuanmao

test/run_test.py

@@ -64,13 +64,11 @@
     'function_schema',
 ]
 
-# skip < 3.3 because mock is added in 3.3 and is used in rpc_fork and rpc_spawn
+# skip < 3.3 because mock is added in 3.3 and is used in rpc_spawn
 # skip python2 for rpc and dist_autograd tests that do not support python2
 if PY33:
     TESTS.extend([
-        'rpc_fork',
         'rpc_spawn',
-        'dist_autograd_fork',
         'dist_autograd_spawn',
         'dist_optimizer_spawn',
     ])