Fix checkpointing bug in tabular forecasting

[ethanwharris]

What does this PR do?

The parameters are needed to restore the checkpoint, but pandas data frames can't be pickled in some versions of pandas. This PR changes it to store the first row of data as a dict (inherently serializable) rather than a dataframe.

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[Borda]

do we have a test for this case?

[codecov]

Codecov Report

Merging #995 (752037e) into master (19cf911) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #995   +/-   ##
=======================================
  Coverage   88.40%   88.40%           
=======================================
  Files         250      250           
  Lines       13199    13199           
=======================================
  Hits        11668    11668           
  Misses       1531     1531           

Flag	Coverage Δ
unittests	88.40% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...h/core/integrations/pytorch_forecasting/adapter.py	91.66% <100.00%> (ø)
flash/tabular/forecasting/data.py	94.82% <100.00%> (ø)
flash/tabular/forecasting/model.py	81.81% <100.00%> (ø)

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