Implement flow matching

[kazewong]

There has been recent work showing there is advantage in using flow matching type of algorithm over discrete normalizing flow. It will be useful to implement such a network and benchmark it.

Here are some references:

1. Torch CFM https://github.com/atong01/conditional-flow-matching
2. Lipman https://arxiv.org/pdf/2210.02747
3. ot flow matching https://arxiv.org/pdf/2302.00482
4. Markovian flow matching https://arxiv.org/pdf/2405.14392