Add initial lowering of aten depthwise convolution to tosa.depthwise_conv2d (#1070)

Summary:
aten.convolution is known as depthwise convolution when groups == in_channels and out_channels == K * in_channels. Lower this operation types to tosa.depthwise_conv2d.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1070

Reviewed By: kirklandsign

Differential Revision: D51167247

Pulled By: digantdesai

fbshipit-source-id: bb4a3c3975d39c24dca1f0d925f4fcc5ed4127b2