Rename package tfjs-webgpu to tfjs-backend-webgpu. (#1700)
Rename package tfjs-webgpu to tfjs-backend-webgpu. (#1700)

Just so we don't forget in the future.

INTERNAL

tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs-core