Add groups to Conv1d (#948)

* Add conv1d grouped convs on CPU

* Add GPU support

* Parallelize inside metal kernel

* clenaup

* Update mlx/ops.cpp

Co-authored-by: Awni Hannun <awni.hannun@gmail.com>

* New unfold kernel + remove unused code

* Remove copy and refactor

* Update vjp and reuse steel gemm

* Fixed groups on cpu

* Fix metal validation

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Co-authored-by: Awni Hannun <awni.hannun@gmail.com>

mlx/array.h

@@ -114,6 +114,15 @@ class array {
     return array_desc_->strides;
   };
 
+  /**
+   *  Get the stride of the corresponding dimension.
+   *
+   *  This function supports negative indexing and provides
+   *  bounds checking. */
+  size_t strides(int dim) const {
+    return strides().at(dim < 0 ? dim + ndim() : dim);
+  };
+
   /** Get the arrays data type. */
   Dtype dtype() const {
     return array_desc_->dtype;