misc updates to fake fp16 tests (#39405)
Summary:
Misc updates to the fake FP16 tests.
1. seeding numpy with a random seed
2. test base class changed from unittest.TestCase=>serial.SerializedTestCase
3. Removed the hypothesis_test_util import
Reviewer: Hector Yuen
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39405
Test Plan: Fake FP16 test
Differential Revision: D21890212
Pulled By: hyuen
fbshipit-source-id: 25e7e17f118655f32cdd06ea9db3cdac5277e649
