example: minGPT add dtype support

[iejMac]

original PR: karpathy/minGPT#86

[iejMac]

openai can you check this out?

[github-actions]

This pull request adds dtype support to the example fake_minGPT model in the model.py file. Tests for the change are added to the test_modeling_gpt2.py file. The changes make it easier to specify the desired dtype when initializing the model and include checks on the specified dtype. A get_memory_footprint() function is also added which will return the memory footprint of the current model in bytes and allows for the benchmarking of the memory footprint of the model and designing tests.

One possible improvement would be to add a comment in the model.py file to explain the purpose of the dtype. This would help future developers better understand the code. Additionally, it may be useful to add more tests to the test_modeling_gpt2.py file to more thoroughly test the new dtype support.