Fix NStepBatchSampler sampling out of bounds indicies

[ludvigk]

When stacksize > 1, NStepBatchSampler samples inds < stacksize, which causes out of bounds errors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.23%. Comparing base (29a6a3e) to head (4faf656).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #74   +/-   ##
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  Coverage   74.23%   74.23%           
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  Files          18       18           
  Lines         850      850           
=======================================
  Hits          631      631           
  Misses        219      219           

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[jeremiahpslewis]

@ludvigk Thanks! Can you please add a test that confirms the fix works?

[ludvigk]

When updating the test it seemed to me as if the episode buffer has a size of capacity + 2. After pushing 12 or more states to the buffer with capacity 10, valid_range returns an array with 12 elements instead of 11. I'm not 100% sure where the extra +1 comes from though.

Clearly I broke something else, so my "fix" was not correct.