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The `GPTWeightLoader` was structured like this in pseudocode:

if marlin:
  Set up tensors in a way that GPTQ-Marlin expects
else:
  Set up tensors in a way that ExLlama/GPTQ/AWQ expect

However, the GPT-Marlin implementation details should really be in the
`marlin` module. So move the former part out to a separate
`GPTQMarlinWeightsLoader`.

The marlin.py file was getting large, split it up.