Clean up and fix primal type to tangent type mapping

This is part of the ["stackless"](#23299) change. I'm splitting it out into a separate PR because we need it for some work on sharding types.

Changes:
  1. Rename `at_least_vspace` to `to_tangent_type` since that's what we always meant by it. `at_least_vspace` was always a bad name (sorry!) but it makes even less sense when you can have a special tangent type for a primal types that's already a vector space itself.
  2. Replace `Zero.from_value` with `Zero.from_primal_value`, which does the required primal-type-to-tangent-type conversion.
  3. Add `to_tangent_type` calls in various other places they're missing.
  4. Remove non-support for float0 in custom deriviatives?
  5. [Optional, WIP] Reinstate some checks that had been skipped over, presumably because of these bugs. (We'll see how far I get with it. Might end up being a separate PR.)
PiperOrigin-RevId: 676115753

distrax/_src/utils/math.py

@@ -14,10 +14,13 @@
 # ==============================================================================
 """Utility math functions."""
 
+import functools
 from typing import Optional, Tuple
 
 import chex
 import jax
+from jax import core as jax_core
+from jax.custom_derivatives import SymbolicZero
 import jax.numpy as jnp
 
 Array = chex.Array
@@ -44,16 +47,36 @@ def multiply_no_nan(x: Array, y: Array) -> Array:
   return jnp.where(y == 0, jnp.zeros((), dtype=dtype), x * y)
 
 
-@multiply_no_nan.defjvp
+# TODO(dougalm): move helpers like these into JAX AD utils
+def add_maybe_symbolic(x, y):
+  if isinstance(x, SymbolicZero):
+    return y
+  elif isinstance(y, SymbolicZero):
+    return x
+  else:
+    return x + y
+
+
+def scale_maybe_symbolic(result_aval, tangent, scale):
+  if isinstance(tangent, SymbolicZero):
+    return SymbolicZero(result_aval)
+  else:
+    return tangent * scale
+
+
+@functools.partial(multiply_no_nan.defjvp, symbolic_zeros=True)
 def multiply_no_nan_jvp(
     primals: Tuple[Array, Array],
     tangents: Tuple[Array, Array]) -> Tuple[Array, Array]:
   """Custom gradient computation for `multiply_no_nan`."""
   x, y = primals
   x_dot, y_dot = tangents
   primal_out = multiply_no_nan(x, y)
-  tangent_out = y * x_dot + x * y_dot
-  return primal_out, tangent_out
+  primal_aval = jax_core.raise_to_shaped(jax_core.get_aval(primal_out))
+  result_aval = primal_aval.at_least_vspace()
+  tangent_out_1 = scale_maybe_symbolic(result_aval, x_dot, y)
+  tangent_out_2 = scale_maybe_symbolic(result_aval, y_dot, x)
+  return primal_out, add_maybe_symbolic(tangent_out_1, tangent_out_2)
 
 
 @jax.custom_jvp