Move codegen from using `torchgen` to `executorch.codegen`

[larryliu0820]

🚀 The feature, motivation and pitch

We already moved torchgen.executorch (in pytorch/pytorch) to executorch.codegen (in pytorch/executorch).

In Codegen.cmake we are still using torchgen.gen_executorch so now it's time to change it to executorch.codegen.

Notice that at this stage we haven't installed executorch so we might need to run the script in a different way (e.g., from executorch.codegen import gen may not work since executorch is not installed yet).

Alternatives

No response

Additional context

No response

RFC (Optional)

No response

cc @mergennachin @byjlw