[TextGeneration] Split up prep_for_generation operator, handle edge cases, handle kv_cache full during prefill

[dsikka]

Summary

A series of improvements:

• Split up the prep_for_generation operator such that we aren't setting up the tokenizer and generating tokens in the same operator, outside of the generation loop
• With this split, make sure we properly handle edge cases, such as max_length=1, max_new_tokens=0, max_new_tokens=1. Also added the condition such that max_length has to be greater than 0 or we throw a ValueError
• With splitting this up, we no longer have to import the process_output operator in the prep_for_generation operator and can remove the custom streaming code in the operator . Also, the non-kv cache and kv_cache pipelines now both share the same output_schema, simplifying generation logic
• Also added a condition to check kv_cache capacity during prompt inference and exit if the capacity is reached before generation

Testing

• Added new tests to validate the edge cases
• Added a test to check if we throw an error when the cache capacity is filled during prefill
• Updated the non-kv cache unit test to also compare logits
• All other test cases pass