Init_tile takes memref not base address (i64) for load gather and sca…

…tter (#930)

docs/rfcs/XeTile.md

@@ -220,10 +220,13 @@ With the data being presented as 4D vector, all the vector based XeTile operatio
 The tile_pack and tile_unpack operation is similar to pack and unpack operation of tensor dialect. The source vector must be a 2D dimension vector, and no permutation is allowed for the result 4D vector, so effectively the blocking effect is identical to tensor pack/unpack operation with inner_dims_pos = [0,1] inner_dims_pos = [0, 1].
 
 ## support for load_gather and store_scatter (experimental)
-`init_tile` can create a tile with each element's address being explictly specified. The tile is created with a base address and offsets for all elements to be loaded. The result tile has a `scatter` attribute to distinguish it from the regular tile.
+`init_tile` can create a tile with each element's address being explictly specified. The tile is created with a base memref and offsets for all elements to be loaded. The offsets and result tile can be either 1D or 2D. The resule tile has a `scatter` attribute to distinguish it from the regular tile.
 ```mlir
-  %tile0 = xetile.init_tile %base_addr, %tile_offsets:
-     i64, vector<1x256xindex> into tile<1x256xbf16, #scatter>
+  %tile0 = xetile.init_tile %base_memref, %tile_offsets:
+     memref<?xbf16>, vector<256xindex> into tile<256xbf16, #scatter>
+
+  %tile0 = xetile.init_tile %base_memref, %tile_offsets:
+     memref<?xbf16>, vector<8x64xindex> into tile<8x64xbf16, #scatter>
 ```
 `load_gather` (aka. load) loads data with prepared tile and mask. Attribute `padding` specifies the padding value for the out-of-boundary access. The default value is zero.
 ```mlir