Make the batch_idx argument optional in all step methods

[0x404]

What does this PR do?

make batch_idx argument optional in test_step, validation_step and predict_step.

Fixes #18506

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📚 Documentation preview 📚: https://pytorch-lightning--18512.org.readthedocs.build/en/18512/

[codecov]

Codecov Report

Merging #18512 (2c0cc5a) into master (4dfc09c) will decrease coverage by 35%.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #18512      +/-   ##
==========================================
- Coverage      85%      50%     -35%     
==========================================
  Files         427      419       -8     
  Lines       33372    33240     -132     
==========================================
- Hits        28373    16703   -11670     
- Misses       4999    16537   +11538     

[carmocca]

LGTM!