#tf-data Re-use `GlobalShuffleIterator` for the Bag dataset.
#tf-data Re-use `GlobalShuffleIterator` for the Bag dataset.

Added an `AnyContext` class for use cases that need to pass
in either an `OpKernelContext` or `IteratorContext`.

For the `BagDataset::Get`, it needs to access `ctx.allocator`
from either `OpKernelContext` or `IteratorContext`.
