Implementation of mlx.random.multivariate_normal (#502) (#877)

* Implementation of mlx.random.multivariate_normal (#502)

* Update python/src/random.cpp

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

* Update python/src/random.cpp

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

* Update python/src/random.cpp

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

* Updated typo in docstring

* Restricted multivariate_normal to  float32

* Generic mean and variance shapes

* Review edits

* Update mlx/random.cpp

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

* Update python/src/random.cpp

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

* Update python/src/random.cpp

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

* Update python/src/random.cpp

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

* Test for ndim of mean and cov

* nits

* smaller size for test

* fix broadcasted sampling

---------

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

mlx/random.h

@@ -121,6 +121,15 @@ inline array normal(
   return normal(shape, float32, 0.0, 1.0, key, s);
 }
 
+/** Generate samples from a multivariate normal distribution. **/
+array multivariate_normal(
+    const array& mean,
+    const array& cov,
+    const std::vector<int>& shape,
+    Dtype dtype,
+    const std::optional<array>& key = std::nullopt,
+    StreamOrDevice s = {});
+
 /** Generate integer samples uniformly at random */
 array randint(
     const array& low,