Add allocate_temp method to KernelRuntimeContext (#3209)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3209

This adds an `allocate_temp` method to KernelRuntimeContext, and passes the temporary memory allocator from `execute_instruction`. The method returns a result that errors if the temporary `MemoryAllocator` was not provided or the memory could not be allocated.

Reviewed By: dbort

Differential Revision: D56421957

fbshipit-source-id: 6da73bdb8e31638fc6d575e98cfc08c27b25f09c