Add fluent methods mean, std, var for ndarray (#16077)

* Add fluent methods mean, std, var for ndarray

* Fix sanity

python/mxnet/numpy/multiarray.py

@@ -1170,11 +1170,15 @@ def nanprod(self, *args, **kwargs):
 
     def mean(self, axis=None, dtype=None, out=None, keepdims=False):  # pylint: disable=arguments-differ
         """Returns the average of the array elements along given axis."""
-        raise NotImplementedError
+        return mean(self, axis=axis, dtype=dtype, out=out, keepdims=keepdims)
 
     def std(self, axis=None, dtype=None, out=None, ddof=0, keepdims=False):  # pylint: disable=arguments-differ
         """Returns the standard deviation of the array elements along given axis."""
-        return _mx_np_op.std(self, axis=axis, dtype=dtype, ddof=ddof, keepdims=keepdims, out=out)
+        return std(self, axis=axis, dtype=dtype, ddof=ddof, keepdims=keepdims, out=out)
+
+    def var(self, axis=None, dtype=None, out=None, ddof=0, keepdims=None):  # pylint: disable=arguments-differ
+        """Returns the variance of the array elements, along given axis."""
+        return var(self, axis=axis, dtype=dtype, out=out, ddof=ddof, keepdims=keepdims)
 
     def cumsum(self, axis=None, dtype=None, out=None):
         """Return the cumulative sum of the elements along the given axis."""

python/mxnet/symbol/numpy/_symbol.py

@@ -530,12 +530,16 @@ def nanprod(self, *args, **kwargs):
         raise AttributeError('_Symbol object has no attribute nanprod')
 
     def mean(self, axis=None, dtype=None, out=None, keepdims=False):  # pylint: disable=arguments-differ
-        """Convenience fluent method for :py:func:`mean`.
+        """Returns the average of the array elements along given axis."""
+        return mean(self, axis=axis, dtype=dtype, out=out, keepdims=keepdims)
 
-        The arguments are the same as for :py:func:`mean`, with
-        this array as data.
-        """
-        return _npi.mean(self, axis=axis, dtype=dtype, keepdims=keepdims, out=out)
+    def std(self, axis=None, dtype=None, out=None, ddof=0, keepdims=False):  # pylint: disable=arguments-differ,too-many-arguments
+        """Returns the standard deviation of the array elements along given axis."""
+        return std(self, axis=axis, dtype=dtype, ddof=ddof, keepdims=keepdims, out=out)
+
+    def var(self, axis=None, dtype=None, out=None, ddof=0, keepdims=None):  # pylint: disable=arguments-differ,too-many-arguments
+        """Returns the variance of the array elements, along given axis."""
+        return var(self, axis=axis, dtype=dtype, out=out, ddof=ddof, keepdims=keepdims)
 
     def cumsum(self, axis=None, dtype=None, out=None):
         """Return the cumulative sum of the elements along the given axis."""

tests/python/unittest/test_numpy_op.py

@@ -401,7 +401,7 @@ def __init__(self, axis=None, dtype=None, keepdims=False):
             self._keepdims = keepdims
 
         def hybrid_forward(self, F, a, *args, **kwargs):
-            return F.np.mean(a, axis=self._axis, dtype=self._dtype, keepdims=self._keepdims)
+            return a.mean(axis=self._axis, dtype=self._dtype, keepdims=self._keepdims)
 
     def is_int(dtype):
         return 'int' in dtype
@@ -467,7 +467,7 @@ def __init__(self, name, axis=None, dtype=None, keepdims=False, ddof=0):
             self._ddof = ddof
 
         def hybrid_forward(self, F, a, *args, **kwargs):
-            return getattr(F.np, self._name)(a, axis=self._axis, dtype=self._dtype, keepdims=self._keepdims, ddof=self._ddof)
+            return getattr(a, self._name)(axis=self._axis, dtype=self._dtype, keepdims=self._keepdims, ddof=self._ddof)
 
     def is_int(dtype):
         return 'int' in dtype