Add multi_quantization to requirements.txt

[csukuangfj]

Someone reports the following error

./pruned_transducer_stateless6/export.py --jit=1
Traceback (most recent call last):
  File "./pruned_transducer_stateless6/export.py", line 52, in <module>
    from train import get_params, get_transducer_model
  File "/xxx/k2-fsa/icefall/egs/librispeech/ASR/pruned_transducer_stateless6/train.py", line 80, in <module>
    from model import Transducer
  File "/xxx/k2-fsa/icefall/egs/librispeech/ASR/pruned_transducer_stateless6/model.py", line 26, in <module>
    from quantization.prediction import JointCodebookLoss
ModuleNotFoundError: No module named 'quantization'

and he tries to use

pip install quantization

but failed.

This PR fixes that.