Add neural network modules, tests, and test utils

[kevinchern]

No description provided.

[thisac]

Have yet to look at tests. Would this fit better in an nn folder, allowing more torch modules to be further organized into their own files. Can still be kept under the torch.nn namespace.

[VolodyaCO]

Left a question about SkipLinear

[anahitamansouri]

Thanks very much Kevin for the great work. I have left my comments.

[kevinchern]

RE @thisac : restructured the file into (sub)modules by pattern-matching pytorch's structure.
RE @VolodyaCO : configs are now stored recursely for nested models :D. reference for resnet added in the docstring
RE @anahitamansouri : fixed typos and added reference for resnet in docstring

[anahitamansouri]

Thanks very much Kevin for addressing the comments.

[thisac]

Thanks @kevinchern. I feel like the tests/utils and its tests could be somewhat simplified, and maybe cut down a bit, otherwise nothing major.

[kevinchern]

RE @thisac : addressed most comments and left some clarifying questions in comments

[kevinchern]

@thisac updated again based on your feedback on best practices. Thanks!!

1. added more tests for the store_config case and separated them as recommended
2. split linear tests as recommended

[thisac]

Suggested change

	class TestUtils(unittest.TestCase):
	class TestHelperFunctions(unittest.TestCase):

[thisac]

Suggested change

	"""Transforms the input `x` with the modules.
	"""Transforms the input ``x`` with the modules.