metal: implement upsample_nearest1d kernel support

[oglego]

Summary

I noticed that upsample_nearest1d was dropping an unimplemented fallback error for the Metal backend package:

fn upsample_nearest1d(&self, _: &Layout, _: usize) -> Result<Self> {
    crate::bail!("Metal upsample_nearest1d not implemented")
}

This PR introduces metal support for upsample_nearest1d. To develop this I followed the structure of the implementation of upsample_nearest2d.

Changes

• MSL Kernel Implementation: Added the upsample_nearest1d shader code and macro variants inside candle-metal-kernels/src/metal_src/conv.metal.
• Kernel Registry: Registered the new shader symbols inside candle-metal-kernels/src/kernels/convolution.rs so the library can load the kernel.
• Backend Dispatch: Updated candle-core/src/metal_backend/mod.rs to route the 1D upsample operation from the frontend down to the new Metal kernel using the existing set_params! framework.
• Testing: Expanded the test suite inside candle-core/tests/pool_tests.rs by adding a test for upsample_nearest1d.

Test plan

Verified locally on Apple Silicon:

• Formatting & Quality: Workspace runs completely clean through cargo fmt --all and cargo clippy.
• Testing: Tests pass when running the below:

cargo test -p candle-core --test pool_tests --features metal