Showing proper error when csr array is not 2D in shape. (#15242)

* Showing proper error when csr array is not 2D in shape.

* Fixed failing CI

* Nudge to CI

python/mxnet/ndarray/ndarray.py

@@ -2227,6 +2227,10 @@ def tostype(self, stype):
         NDArray, CSRNDArray or RowSparseNDArray
             A copy of the array with the chosen storage stype
         """
+        if stype == 'csr' and len(self.shape) != 2:
+            raise ValueError("To convert to a CSR, the NDArray should be 2 Dimensional. Current "
+                             "shape is %s" % str(self.shape))
+
         return op.cast_storage(self, stype=stype)
 
     def to_dlpack_for_read(self):

tests/python/unittest/test_sparse_ndarray.py

@@ -963,6 +963,11 @@ def test_sparse_nd_check_format():
     indptr_list = [0, -2, 2, 3]
     a = mx.nd.sparse.csr_matrix((data_list, indices_list, indptr_list), shape=shape)
     assertRaises(mx.base.MXNetError, a.check_format)
+    # CSR format should be 2 Dimensional.
+    a = mx.nd.array([1, 2, 3])
+    assertRaises(ValueError, a.tostype, 'csr')
+    a = mx.nd.array([[[1, 2, 3]]])
+    assertRaises(ValueError, a.tostype, 'csr')
     # Row Sparse format indices should be less than the number of rows
     shape = (3, 2)
     data_list = [[1, 2], [3, 4]]