fix lost freq of pandas timestamp

[vafl]

Issue #, if available:

Fixes #78

Description of changes:

Fix lost frequency in pandas timestamp

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[szha]

Job PR-79/1 is complete.
Docs are uploaded to http://gluon-ts-staging.s3-accelerate.dualstack.amazonaws.com/PR-79/1/index.html

[alexw91]

Codecov Report

Merging #79 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage    78.6%   78.61%   +<.01%     
==========================================
  Files         110      110              
  Lines        6278     6279       +1     
==========================================
+ Hits         4935     4936       +1     
  Misses       1343     1343

Impacted Files	Coverage Δ
src/gluonts/transform.py	81.49% <100%> (+0.04%)	⬆️

[lostella]

Is this a bug in pandas?

[benidis]

I reproduced the error downloading the quandl dataset and following the code. I was ready to push the fix and then I saw that Valentin had already fixed it!

I think it is a pandas bug.

BTW m4_monthly and m4_weekly do not run for exactly this reason. This may explain a lot about the overall m4 results.

[vafl]

@lostella Yes, I suspect it broke in a recent version of pandas. I'll check and perhaps submit an issue.