Removed process_position argument from Trainer Class

[shenoynikhil]

What does this PR do?

Part of #12521

resolves #9222

This PR removes the process_position argument in the Trainer Class and the corresponding changes. Made the following edits,

• Removed process_position from Trainer and Callback Connector Class
• Removed the test in the deprecation tests file
• Removed the documentation in trainer.rst

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[rohitgr7]

Neat!