[PyTorch] Don't store multiple kernels per key on mobile (#64447)

[PyTorch] Don't store multiple kernels per key on mobile (#64447)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64447

As the code comment says, we needn't worry about Jupyter notebooks on mobile.
ghstack-source-id: 137951718

Test Plan: Profiled startup of //caffe2/caffe2/fb/high_perf_models/pytorch/benchmark_framework_overheads:cpp_benchmark on devserver with -niter 0 -nrep 0 and `C10_DISPATCHER_ONE_KERNEL_PER_DISPATCH_KEY` defined. Time spent in sherwood_v3_table lookups went way down.

Reviewed By: ezyang, bhosmer

Differential Revision: D30736094

fbshipit-source-id: bcc22cd0d9adceba259a03898c992759d501fe89