Summary: Initial TOSA support in executorch (#161)

Summary:
An implementation of a TOSA Partitioner and TOSA backend supporting a small list of operators in fp32 and int8/32. This includes a set of integer networks which will then compile to target Ethos-U NPU using the Vela compiler.

* A small set of test networks in fp32
* A subset of these supporting integer and compiling with Vela
* A test wrapper to register and test networks through the compile stack
  * A flow pt2 -> executorch -> tosa_flatbuffer.tosa file -> tosa_reference_model
  * A flow pt2 -> executorch -> tosa_flatbuffer.tosa file -> vela -> command_stream
* This work depends on a few different python modules
  * https://review.mlplatform.org/plugins/gitiles/tosa/serialization_lib/
  * https://review.mlplatform.org/plugins/gitiles/ml/ethos-u/ethos-u-vela/
* This work uses other projects for validation
  * https://review.mlplatform.org/plugins/gitiles/tosa/reference_model/

Pull Request resolved: https://github.com/pytorch/executorch/pull/161

Reviewed By: mergennachin

Differential Revision: D49542254

Pulled By: digantdesai

fbshipit-source-id: bb4074d5b66233c85452c2373a08ddc1cdd0826f