[ENH] `TemporalFusionTransformer` - allow mixed precision training

[Marcrb2]

Description

This PR modifies the attention mask in the TFT model from 1e-9 to float("inf") to allow Pytorch mixed precision training.

Closes #1325, closes #285

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (b3fcf86) 90.19% compared to head (d93c1e0) 90.19%. Report is 8 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1518   +/-   ##
=======================================
  Coverage   90.19%   90.19%           
=======================================
  Files          30       30           
  Lines        4724     4724           
=======================================
  Hits         4261     4261           
  Misses        463      463           

Flag	Coverage Δ
cpu	90.19% <100.00%> (ø)
pytest	90.19% <100.00%> (ø)

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