Set the dtype correctly for vision GPT model

[SeanNaren]

What does this PR do?

Closes Lightning-AI/pytorch-lightning#8125

We need to ensure the tokens are in the same type or they will be casted to the default FloatTensor.

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[codecov]

Codecov Report

Merging #694 (5ae8247) into master (ab801aa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #694   +/-   ##
=======================================
  Coverage   72.64%   72.64%           
=======================================
  Files         119      119           
  Lines        7356     7356           
=======================================
  Hits         5344     5344           
  Misses       2012     2012           

Flag	Coverage Δ
cpu	72.64% <100.00%> (ø)
pytest	72.64% <100.00%> (ø)

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

Impacted Files	Coverage Δ
pl_bolts/models/vision/image_gpt/gpt2.py	96.15% <100.00%> (ø)

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[tchaton]

LGTM !