Chapter 1 first training fails with PyTorch 2

[MattTW]

By default, PyTorch 2.x is now getting pulled in by pip install of fastbook and its dependencies.

This results in an error in the first training exercise:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
[<ipython-input-8-94c80e27586d>](https://vf2u0v4yexl-496ff2e9c6d22116-0-colab.googleusercontent.com/outputframe.html?vrz=colab-20230509-060147-RC00_530563781#) in <cell line: 12>()
     10     label_func=is_cat, item_tfms=Resize(224))
     11 
---> 12 learn = vision_learner(dls, resnet34, metrics=error_rate)
     13 learn.fine_tune(1)

10 frames
[/usr/local/lib/python3.10/dist-packages/torch/serialization.py](https://vf2u0v4yexl-496ff2e9c6d22116-0-colab.googleusercontent.com/outputframe.html?vrz=colab-20230509-060147-RC00_530563781#) in persistent_load(saved_id)
   1139             typed_storage = loaded_storages[key]
   1140         else:
-> 1141             nbytes = numel * torch._utils._element_size(dtype)
   1142             typed_storage = load_tensor(dtype, nbytes, key, _maybe_decode_ascii(location))
   1143 

AttributeError: module 'torch' has no attribute '_utils'

This can be resolved by forcing a downgrade back to PyTorch 1.x in the notebook e.g.:

pip install torch==1.13.1 torchvision==0.14.1 torchaudio==0.13.1

Would be good to include a quick troubleshooting note. New users just getting started will be frustrated.