warn users when predict/unbatch output length is not same as #requests

[aniketmaurya]

What does this PR do?

This PR, helps user find the mistake in LitAPI when the output from predict/unbatch is not as the same size as of number of requests.

class ExampleAPI(LitAPI):
    ...
    def predict(self, x):
        return self.model(x)  # returns a list of length same as len(x)

    def unbatch(self, output):
        return [output]   # returns a list of length 1

Also, removes a duplicate code.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

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[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88%. Comparing base (43692d4) to head (dedca3e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #408   +/-   ##
===================================
  Coverage    88%    88%           
===================================
  Files        30     30           
  Lines      1976   1946   -30     
===================================
- Hits       1734   1715   -19     
+ Misses      242    231   -11