Investigating CompositeExplicitAugograd dispatch key

[xytintel]

🚀 The feature, motivation and pitch

Whether CompositeExplicitAugograd codegen flag is needed requires further investigation.

Alternatives

No response

Additional context

No response

[xytintel]

• CompositeExplicitAutograd (previously known as DefaultBackend):
  implementations of kernels that work for all backends, but require an
  explicit definition of backward function in derivatives.yaml to support autograd.
  The most typical use of this key are for delegating functions; i.e.,
  functions that do a very small amount of work and then delegate to another
  operator to do the actual heavy lifting. Under the hood, registering a
  kernel to CompositeExplicitAutograd is equivalent to registering that
  kernel to every backend (e.g., CPU, CUDA). Note: kernels which call
  DispatchStub should NOT be registered as CompositeExplicitAutograd, as
  DispatchStub only works for CPU, CUDA)