Mark Lee

An integration of orbax checkpointer.

1

Depends on https://github.com/apple/axlearn/pull/650. The relevant changes are in `checkpointer.py`, `checkpointer_test.py`, and `pyproject.toml`. It also depends on an unreleased commit for orbax (for concurrency bounded serialization).

### Description A simple repro: ``` import jax.ad_checkpoint from jax.ad_checkpoint import checkpoint_policies as jax_remat_policies from jax.experimental.attrs import jax_getattr, jax_setattr class A: ... a = A() jax_setattr(a, 'x', 0) def fn(c):...