First commit of scratch jax2dex file.
First commit of scratch jax2dex file.

We may have left this in a broken state when we started integrating JAX dynamic
shape features.

Co-authored-by: Dougal Maclaurin <0bb64e81bea11b43bfc7c06c1607c3af64f31e6d@google.com>

google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang