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)

[xytintel]

The conclusion is that we don't need it.