[FEATURE] Upgrade fastai to v2

[SimonYansenZhao]

Description

fastai has breaking changes in v2, which makes the code in recommenders incompatible with fastai v2. For example, CollabDataBunch is removed in fastai v2.

Expected behavior with the suggested feature

It should be compatible with fastai v2 because fastai v1 is not maintained.

Other Comments

There are 3 repos related to fastai:

• fastai1: fastai v1
• fastai2: fastai v2 (archived)
• fastai: fastai v2+

See also

• Fastai V2 Upgrade Review Guide
• https://github.com/recommenders-team/recommenders/pull/1988: https://github.com/recommenders-team/recommenders/pull/1988/commits/97deed179525b2f8bc6d4f68f973e7cfb9676f6f