Aten _To_Copy (#6055)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6055

Implement aten._to_copy. Currently we are only interested in fp32 <-> fp16 conversions, but it should theoritically support other dtype conversions too. I noticed an issue with int conversions so limited it to just fp32 and fp16 for now.

Note: Most driver implementations of fp16 cast does not "round up" the result, therefore there might be 1 bit difference between vulkan output and cpu torch.to. Explained in greater detail in the comments.

Reviewed By: SS-JIA

Differential Revision: D64080303

fbshipit-source-id: d019fae8269a2ea53991f5e6f81a3fe209ee09d0