Metal commands refactoring (#2489)

* Split out the commands part of the metal device.

* Make most fields private.

* Move the allocator back.

* Rework the encoder provider type.

candle-metal-kernels/src/utils.rs

@@ -168,17 +168,22 @@ pub trait EncoderProvider {
     fn encoder(&self) -> Self::Encoder<'_>;
 }
 
-pub struct WrappedEncoder<'a>(&'a ComputeCommandEncoderRef);
+pub struct WrappedEncoder<'a> {
+    inner: &'a ComputeCommandEncoderRef,
+    end_encoding_on_drop: bool,
+}
 
 impl<'a> Drop for WrappedEncoder<'a> {
     fn drop(&mut self) {
-        self.0.end_encoding()
+        if self.end_encoding_on_drop {
+            self.inner.end_encoding()
+        }
     }
 }
 
 impl<'a> AsRef<metal::ComputeCommandEncoderRef> for WrappedEncoder<'a> {
     fn as_ref(&self) -> &metal::ComputeCommandEncoderRef {
-        self.0
+        self.inner
     }
 }
 
@@ -187,7 +192,10 @@ impl EncoderProvider for &metal::CommandBuffer {
     where
         Self: 'a;
     fn encoder(&self) -> Self::Encoder<'_> {
-        WrappedEncoder(self.new_compute_command_encoder())
+        WrappedEncoder {
+            inner: self.new_compute_command_encoder(),
+            end_encoding_on_drop: true,
+        }
     }
 }
 
@@ -196,6 +204,21 @@ impl EncoderProvider for &metal::CommandBufferRef {
     where
         Self: 'a;
     fn encoder(&self) -> Self::Encoder<'_> {
-        WrappedEncoder(self.new_compute_command_encoder())
+        WrappedEncoder {
+            inner: self.new_compute_command_encoder(),
+            end_encoding_on_drop: true,
+        }
+    }
+}
+
+impl EncoderProvider for &ComputeCommandEncoderRef {
+    type Encoder<'a> = WrappedEncoder<'a>
+    where
+        Self: 'a;
+    fn encoder(&self) -> Self::Encoder<'_> {
+        WrappedEncoder {
+            inner: self,
+            end_encoding_on_drop: false,
+        }
     }
 }