Move codegen from using torchgen to executorch.codegen #11232
Description
🚀 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
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Additional context
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RFC (Optional)
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