Raise a warning if evaulation is triggered with best ckpt in case of multiple checkpoint callbacks

[rohitgr7]

What does this PR do?

Now that multiple checkpoints are supported, evaluation with the best ckpt cannot be interpreted in such a case. So an error will be raised asking users to specify the exact ckpt path.

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Part of #1

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cc @Borda @awaelchli @rohitgr7

[codecov]

Codecov Report

Merging #11274 (8e49ee4) into master (a610e04) will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #11274   +/-   ##
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  Coverage      92%      92%           
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  Files         194      194           
  Lines       16965    16965           
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  Hits        15593    15593           
  Misses       1372     1372