Set max kernel num (#204)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/204

Currently, max number of kernels is [hardcoded in operator_registry.h](https://github.com/pytorch/executorch/blob/main/runtime/kernel/operator_registry.h). This diff adds a preprocessor flag to set the max kernel num to save binary size.

Reviewed By: larryliu0820

Differential Revision: D48922452

fbshipit-source-id: a3d3181ba9063b0dc8f89f94d033f5f884076861