Improve documentation and warning message for creation of a tensor wi…

…th from_numpy() (#49516)

Summary:
Implements very simple changes suggested in the short discussion of the issue. Updated documentation to inform user that creation of tensor with memory mapped read only numpy arrays will probably cause a crash of the program. The displayed warning message was also updated to contain the information about issues concerning the use of a memory mapped read only numpy array. Closes #46741.

Pull Request resolved: #49516

Reviewed By: mrshenli

Differential Revision: D25746115

Pulled By: mruberry

fbshipit-source-id: 3e534a8f2bc1f083a2835440d324bd6f30798ad4

torch/_torch_docs.py

@@ -3080,6 +3080,9 @@ def merge_dicts(*dicts):
 ``numpy.int64``, ``numpy.int32``, ``numpy.int16``, ``numpy.int8``, ``numpy.uint8``,
 and ``numpy.bool``.
 
+.. warning::     
+    Writing to a tensor created from a read-only NumPy array is not supported and 
+    will result in undefined behavior.
 Example::
 
     >>> a = numpy.array([1, 2, 3])

torch/csrc/utils/tensor_numpy.cpp

@@ -136,11 +136,12 @@ at::Tensor tensor_from_numpy(PyObject* obj, bool warn_if_not_writeable/*=true*/)
   if (!PyArray_ISWRITEABLE(array) && warn_if_not_writeable) {
     TORCH_WARN_ONCE(
       "The given NumPy array is not writeable, and PyTorch does "
-      "not support non-writeable tensors. This means you can write to the "
-      "underlying (supposedly non-writeable) NumPy array using the tensor. "
-      "You may want to copy the array to protect its data or make it writeable "
-      "before converting it to a tensor. This type of warning will be "
-      "suppressed for the rest of this program.");
+      "not support non-writeable tensors. Writing to this tensor is not "
+      "supported and will result in undefined behavior. "
+      "You may want to copy the "
+      "array to protect its data or make it writeable before converting it to " 
+      "a tensor. "
+      "This type of warning will be suppressed for the rest of this program.");
 
   }