1. Move distribution strategy context utility methods to a separate file with...

1. Move distribution strategy context utility methods to a separate file with few dependencies. This allows us to import this in some places without creating circular dependencies as the original file imported many things.
2. Move the stack used in distribution strategy context to the graph. This allows us to use different strategies in different graphs (for e.g. in train and eval).
This fixes #21412 and #21180.

PiperOrigin-RevId: 208680454