[JAX SC] Use private _mesh field and public mesh property in SparseCoreEmbed

* Replace dataclass `mesh` field with private `_mesh` and `mesh` property.
* Update callers to pass `_mesh`.

PiperOrigin-RevId: 973968115

Author: adityagupta1089

recml/inference/models/jax/DLRM_DCNv2/dlrm_model.py

@@ -135,7 +135,7 @@ def __call__(
 
     sparse_embeddings = embed.SparseCoreEmbed(
         feature_specs=self.feature_specs,
-        mesh=self.mesh,
+        _mesh=self.mesh,
         sharding_axis=self.sharding_axis,
     )(embedding_lookups)
     sparse_embeddings = jax.tree.flatten(sparse_embeddings)