Reland: Remove remaining global `set_default_dtype` calls from tests (#108088)
Fixes #68972
Relands #107246
To avoid causing Meta-internal CI failures, this PR avoids always asserting that the default dtype is float in the `TestCase.setUp/tearDown` methods. Instead, the assert is only done if `TestCase._default_dtype_check_enabled == True`. `_default_dtype_check_enabled` is set to True in the `if __name__ == "__main__":` blocks of all the relevant test files that have required changes for this issue
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108088
Approved by: https://github.com/ezyang
