Add Torch Unit Test Suite to Reference Model

Unit test test suite for PyTorch utilizing FxImporter to lower PyTorch
graphs to Torch-MLIR then to TOSA IR. Current infrastructure is mostly
based on the existing TF/TFL frameworks test suite.

This is the initial work for the Torch unit test suite. The end goal
is to add more Torch unit tests for a more robust test suite.

Signed-off-by: Justin Ngo <justin.ngo@arm.com>
Change-Id: I829ddf708556488166bc2bfdd4b52a1cfb4fc34d