Improve quantize_comm error message (pytorch#2018)

Summary:
Pull Request resolved: pytorch#2018

As titled

Reviewed By: jianyuh, henrylhtsang, edqwerty10

Differential Revision: D49295738

fbshipit-source-id: 45524d8e220ba6b686a99d201e24c6a3d839aed7

fbgemm_gpu/fbgemm_gpu/quantize_comm.py

@@ -193,9 +193,10 @@ def calc_quantized_size(
             self._comm_precision == SparseType.FP8 and self._row_dim > 0
         ):
             ctx = none_throws(ctx)
-            assert (
-                input_len % ctx.row_dim == 0
-            ), f"input_len {input_len} is not a multiple of row dim {ctx.row_dim}"
+            assert input_len % ctx.row_dim == 0, (
+                f"input_len {input_len} is not a multiple of row dim {ctx.row_dim} "
+                "Please check your batch size (power of 2 batch size is recommended)"
+            )
             nrows = input_len // ctx.row_dim
             ncols = (ctx.row_dim + 3) // 4 * 4 + 2 * 4
             return nrows * ncols