[Pallas] Switch gather to jnp.take_along_axis (for JAX issue filing)

This version uses jnp.take_along_axis which is the natural JAX equivalent
of torch.gather. It works in interpret mode but fails on real TPU due to
a limitation in Mosaic's lax.gather lowering rule which requires
indices.shape == input.shape + (1,).

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

stack-info: PR: #2061, branch: AmesingFlank/stack/27

Author: AmesingFlank

helion/_compiler/aten_lowering.py

@@ -1767,12 +1767,11 @@ def codegen_gather(ctx: LoweringContext, node: Node) -> object:
 
 @gather_lowering.register_codegen("pallas")
 def codegen_gather_pallas(ctx: LoweringContext, node: Node) -> object:
-    """Generate gather for Pallas using one_hot + multiply + sum.
+    """Generate gather for Pallas using jnp.take_along_axis.
 
-    TPU Mosaic has limited lax.gather support, so we implement
-    gather(input, dim, index) as:
-        mask = one_hot(index.squeeze(dim), input.shape[dim], dtype=input.dtype)
-        result = sum(input * mask, axis=dim, keepdims=True)
+    This is the natural JAX equivalent of torch.gather but currently fails
+    on TPU Mosaic due to limited lax.gather lowering support.
+    See: https://github.com/jax-ml/jax/issues/XXXXX
     """
     assert not node.kwargs, "gather does not support keyword arguments"
     assert len(node.args) == 3, f"gather expects 3 arguments, got {len(node.args)}"
@@ -1805,35 +1804,13 @@ def codegen_gather_pallas(ctx: LoweringContext, node: Node) -> object:
     index_ast = _env_arg(ctx, index_node)
     assert isinstance(index_ast, ast.AST)
 
-    idx_var = fn.new_var("gather_idx")
-    mask_var = fn.new_var("gather_mask")
     result_var = fn.new_var("gather_result")
 
     ctx.cg.add_statement(
         statement_from_string(
-            f"{idx_var} = jnp.squeeze({{index}}.astype(jnp.int32), axis={dim})",
-            index=index_ast,
-        )
-    )
-
-    ctx.cg.add_statement(
-        statement_from_string(
-            f"{mask_var} = jax.nn.one_hot({idx_var}, {{input}}.shape[{dim}], dtype={{input}}.dtype)",
-            input=input_ast,
-        )
-    )
-
-    if dim != ndim - 1:
-        ctx.cg.add_statement(
-            statement_from_string(
-                f"{mask_var} = jnp.moveaxis({mask_var}, -1, {dim})",
-            )
-        )
-
-    ctx.cg.add_statement(
-        statement_from_string(
-            f"{result_var} = jnp.sum({{input}} * {mask_var}, axis={dim}, keepdims=True)",
+            f"{result_var} = jnp.take_along_axis({{input}}, {{index}}.astype(jnp.int32), axis={dim})",
             input=input_ast,
+            index=index_ast,
         )
     )