Reduce peak memory usage when freezing parameters.

[lkhphuc]

I discovered a optax.set_to_zero() from this thread.

When compare with the original optax.scale(0.0) on a ViT H/16 with some heads, peak GPU memory usage (by setting XLA_PYTHON_CLIENT_PREALLOCATE=false):

• Full trainable: 18GiB
• optax.scale(0.0) (current): 9.8GiB
• optax.set_to_zero (PR): 5.6GiB

The frozen weight was set in the config like this (for both current and PR change):

  config.schedule = [
    (".*ViT_0/.*", None),
    (".*", dict(warmup_steps=2500))
  ]

Theoretically memory usage should be the same after jitted, so I'm not sure if this is a GPU-specific bug from jax or not.