implicit BPR-MF wrapper model

[blondered]

Feature Description

Create a wrapper for popular BPR-MF baseline

Why this feature?

It's easy to implement and it's often used as a baseline in research

Additional context

No response

[chezou]

Created an experimental PR. #232

I know there are several discussion points, but I hope it clarifies the desired specs.

Currently, the PR excludes training with features.

I think support fit with features for GPU is easier than CPU because preparation before implicit.gpu.bpr_update() is written in Python https://github.com/benfred/implicit/blob/b33b809cb585cb8a65ad39d0f97497d37e98acaa/implicit/gpu/bpr.py#L137 while implicit.cpu.bpr implements in Cython. https://github.com/benfred/implicit/blob/b33b809cb585cb8a65ad39d0f97497d37e98acaa/implicit/cpu/bpr.pyx#L137-L187