Add support for max_pool2d for Metal backend (#1863)

* first pass at implementation of maxpool2d

* Add definitions for other dtypes

* add tests for other dtypes

* Cosmetic tweaks + re-enable maxpool2d tests for metal.

---------

Co-authored-by: Laurent <laurent.mazare@gmail.com>

candle-metal-kernels/src/lib.rs

@@ -1826,5 +1826,38 @@ fn divide(m: usize, b: usize) -> NSUInteger {
     ((m + b - 1) / b) as NSUInteger
 }
 
+#[allow(clippy::too_many_arguments)]
+pub fn call_max_pool2d(
+    device: &Device,
+    command_buffer: &CommandBufferRef,
+    kernels: &Kernels,
+    name: &'static str,
+    shape: &[usize],
+    strides: &[usize],
+    out_w: usize,
+    out_h: usize,
+    w_k: usize,
+    h_k: usize,
+    w_stride: usize,
+    h_stride: usize,
+    input: &Buffer,
+    output: &Buffer,
+) -> Result<(), MetalKernelError> {
+    let dst_el = out_w * out_h * shape[0] * shape[1];
+    let pipeline: ComputePipelineState = kernels.load_pipeline(device, Source::Conv, name)?;
+    let (thread_group_count, thread_group_size) = linear_split(&pipeline, dst_el);
+    let encoder = command_buffer.new_compute_command_encoder();
+    encoder.set_compute_pipeline_state(&pipeline);
+    set_params!(
+        encoder,
+        (w_k, h_k, w_stride, h_stride, shape, strides, input, output)
+    );
+    encoder.use_resource(input, metal::MTLResourceUsage::Read);
+    encoder.use_resource(output, metal::MTLResourceUsage::Write);
+    encoder.dispatch_thread_groups(thread_group_count, thread_group_size);
+    encoder.end_encoding();
+    Ok(())
+}
+
 #[cfg(test)]
 mod tests;