Enable batch-unbatch by default

[aniketmaurya]

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

This PR makes LitAPI.batch and LitAPI.unbatch enable by default where it returns the input and output as list if not implemented.

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 82%. Comparing base (144476b) to head (429e014).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #220   +/-   ##
===================================
+ Coverage    80%    82%   +1%     
===================================
  Files        13     13           
  Lines      1057   1048    -9     
===================================
+ Hits        850    855    +5     
+ Misses      207    193   -14     

[lantiga]

Looks great.

Caveat: this

LitServe/src/litserve/api.py

Line 24 in 144476b

def no_batch_unbatch_message_no_stream(obj, data):

and

LitServe/src/litserve/api.py

Line 40 in 144476b

def no_batch_unbatch_message_stream(obj, data):

are now both unused and the error message is inaccurate. We could just remove.

At first I thought we could catch an exception but the error from incorrect batching would show up in predict and encode_response, so I don't think we can catch it without producing confusing false positives.