Enforce strides to be not null when ndim is nonzero (#178)

* Enforce strides to be not null when ndim is nonzero

This PR updates the spec to enforce the strides to be not null
when the ndim is nonzero.
This is going to help consumers to handle cases more uniformly.

* Add more comments on the null rationale

* Remove mention of framework handling

* Update NULL note, bump minor

Author: tqchen

include/dlpack/dlpack.h

@@ -19,7 +19,7 @@
 #define DLPACK_MAJOR_VERSION 1
 
 /*! \brief The current minor version of dlpack */
-#define DLPACK_MINOR_VERSION 1
+#define DLPACK_MINOR_VERSION 2
 
 /*! \brief DLPACK_DLL prefix for windows */
 #ifdef _WIN32
@@ -254,11 +254,23 @@ typedef struct {
   int32_t ndim;
   /*! \brief The data type of the pointer*/
   DLDataType dtype;
-  /*! \brief The shape of the tensor */
+  /*!
+   * \brief The shape of the tensor
+   *
+   *  When ndim == 0, shape can be set to NULL.
+   */
   int64_t* shape;
   /*!
-   * \brief strides of the tensor (in number of elements, not bytes)
-   *  can be NULL, indicating tensor is compact and row-majored.
+   * \brief strides of the tensor (in number of elements, not bytes),
+   *  can not be NULL if ndim != 0, must points to
+   *  an array of ndim elements that specifies the strides,
+   *  so consumer can always rely on strides[dim] being valid for 0 <= dim < ndim.
+   *
+   *  When ndim == 0, strides can be set to NULL.
+   *
+   *  \note Before DLPack v1.2, strides can be NULL to indicate contiguous data.
+   *        This is not allowed in DLPack v1.2 and later. The rationale
+   *        is to simplify the consumer handling.
    */
   int64_t* strides;
   /*! \brief The offset in bytes to the beginning pointer to data */