Expose a function to update exllama max input length

[fxmarty]

Exllama requires to set a buffer on the C++ side in the act-order case, so as to reorder the activations. Although by default this buffer is of length 2048, we provide an API to allow to change the size of this buffer:

from auto_gptq import exllama_set_max_input_length
model = exllama_set_max_input_length(model, 4096)

Fixes #253

Test with: CUDA_VISIBLE_DEVICES=0 pytest tests/test_q4.py -k "test_exllama_buffer_size" -s

@PanQiWei I think it would be worth a patch release, what do you think?

[PanQiWei]

Thank you very much for this pr @fxmarty !