Adding Function Return Type + minor code change

[Aniket-Mishra]

Added return type to functions that were missing it in timegpt.py Found an issue mentioned here during the same: https://github.com/Nixtla/nixtla/issues/94

We were doing the size calculation twice in line 356 and line 361. Replaced it with a variable instead.

[CLAassistant]

All committers have signed the CLA.

[AzulGarza]

Hey @Aniket-Mishra! Thank you! In this repo, we use nbdev to generate nixtlats/timegpt.py. Here's a step-by-step guide. Could you update the PR following the tutorial there? Thank you!

[mergenthaler]

This is solved by the new release.