Fix casual mask for transformer decoder

[LongxingTan]

Description

This PR ...

Checklist

• Linked issues (if existing)
• Amended changelog.md for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Thank you for joining. Have fun coding!

[codecov]

Codecov Report

Patch coverage: 96.29% and project coverage change: +0.42 🎉

Comparison is base (0bdd609) 87.79% compared to head (75f6d07) 88.22%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   87.79%   88.22%   +0.42%     
==========================================
  Files          27       27              
  Lines        2114     2114              
==========================================
+ Hits         1856     1865       +9     
+ Misses        258      249       -9     

Flag	Coverage Δ
cpu	88.22% <96.29%> (+0.42%)	⬆️
unittest	88.22% <96.29%> (+0.42%)	⬆️

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

Impacted Files	Coverage Δ
tfts/models/transformer.py	88.29% <94.11%> (+0.05%)	⬆️
examples/run_prediction.py	93.02% <100.00%> (ø)
tfts/layers/attention_layer.py	89.93% <100.00%> (+0.67%)	⬆️
tfts/layers/mask_layer.py	100.00% <100.00%> (+28.57%)	⬆️
tfts/models/informer.py	90.18% <100.00%> (+0.18%)	⬆️

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.