Description
Implements Pad::vmap and removes the NYI path in mlx/primitives.cpp.

Changes

• Implement Pad::vmap in core.
• Remap pad axes when vmapping over the input axis (including negative pad axes).
• Keep in_axes=(0, None) behavior for scalar constant_values.
• Reject vmapping over the pad value (axes[1] >= 0) with invalid_argument (Python ValueError).
• Add mx.pad vmap coverage in python/tests/test_vmap.py.

Tests added

• in_axes=0
• in_axes=1
• in_axes=-1, out_axes=-1
• nested vmap
• in_axes=(0, None) for pad value
• mapped pad value raises ValueError

No change to existing pad semantics outside vmap support.
Closes the NYI stub at mlx/primitives.cpp:3234.