[Tracker] Migrate test classes to use BaseTester and its functionalities.

[johnnv1]

Following on from #2745

Our tests are grouped using classes for tests. In the past, we could not adopt our BaseTester everywhere, because it had some abstract methods which made it half annoying. With our kinda "new" BaseTester - from testing.base import BaseTester - we can adopt it everywhere.

What needs to be done:

1. pickup a test file
2. add the BaseTester as the parent class of the test classes
3. update the assert_close to be self.assert_close
4. update the test_gradcheck to use self.gradcheck instead of gradcheck from torch.autograd
5. delete the use of tensor_to_gradcheck_var on the test_gradcheck - this is automatically handled by the BaseTester class
6. extra: remove the dtype argument of the test_gradcheck and just run it using torch.float64
7. open a PR :)

You can check #2745, which has done a bunch of these updates across our tests

The goal here is to reorganize our test suite. If you have any questions, you can call us and/or ask on our Slack page too