Return zero VJPs for scatter indices

[devin-lai]

Summary

• Return zero VJPs for scatter and scatter_axis index inputs.
• Match gather behavior and fix gradients when scatter indices come from differentiable values.
• Add autograd coverage for direct and derived scatter indices.

Closes #1439.

Tests

• /opt/homebrew/bin/black --check python/tests/test_autograd.py
• clang-format --dry-run --Werror mlx/primitives.cpp
• git diff --check
• PYTHONPATH=/Users/ldy/Desktop/project/mt/opencode/mlx/python:/Users/ldy/Desktop/project/mt/opencode/mlx/python/tests python3.11 -m unittest python.tests.test_autograd.TestAutograd.test_scatter_index_vjp
• PYTHONPATH=/Users/ldy/Desktop/project/mt/opencode/mlx/python:/Users/ldy/Desktop/project/mt/opencode/mlx/python/tests python3.11 -m unittest python.tests.test_autograd

[zcbenz]

Looks good to me, thanks!