[Feature] use extra_requirement to add torch as optional dependency to mlagents (#4417)

* Feature use extra_requirement to add torch as optional dependency to mlagents
 - todo: documentation

* Update ml-agents/setup.py

Co-authored-by: Chris Elion <chris.elion@unity3d.com>

Co-authored-by: Chris Elion <chris.elion@unity3d.com>

ml-agents/setup.py

         ]
     },
     cmdclass={"verify": VerifyVersionCommand},
+    extras_require={"torch": ["torch>=1.5.0"]},
 )