Fix fill_with_mode when using Dask

[arnavgarg1]

Dask returns a Dask index structure because of lazy evaluation, so we need to force computation of the value_counts() method so we can grab the mode of the column to use as the fill_value.

[github-actions]

Unit Test Results

         6 files  ±  0           6 suites  ±0   5h 4m 59s ⏱️ - 18m 51s
  3 890 tests +  2    3 853 ✔️ +  2    37 💤 ±  0  0 ❌ ±0 
11 667 runs  +68  11 556 ✔️ +55  111 💤 +13  0 ❌ ±0 

Results for commit e37e685. ± Comparison against base commit aa88052.

[justinxzhao]

Thanks for the quick fix!