Easy fix for vulkan (#3548)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3548

torchgen is an external dependency. In fbsource, it depends on fbsource/caffe2/...

However in OSS, it depends on pip package.

external_deps is the right way to call it.

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Reviewed By: yipjustin, SS-JIA

Differential Revision: D57120984

fbshipit-source-id: 879f065a2dc5e21ebb5dc2102228e2b7a87a8d2c