update d_kv'annotation in mt5'configuration (#27585)

* update d_kv'annotation in mt5'configuration

* update d_kv'annotation in mt5'configuration

* update d_kv'annotation in mt5'configuration

Author: callanwu

src/transformers/models/mt5/configuration_mt5.py

@@ -40,8 +40,8 @@ class MT5Config(PretrainedConfig):
         d_model (`int`, *optional*, defaults to 512):
             Size of the encoder layers and the pooler layer.
         d_kv (`int`, *optional*, defaults to 64):
-            Size of the key, query, value projections per attention head. `d_kv` has to be equal to `d_model //
-            num_heads`.
+            Size of the key, query, value projections per attention head. In the conventional context, it is typically expected that `d_kv` has to be equal to `d_model // num_heads`.
+            But in the architecture of mt5-small, `d_kv` is not equal to `d_model //num_heads`. The `inner_dim` of the projection layer will be defined as `num_heads * d_kv`.
         d_ff (`int`, *optional*, defaults to 1024):
             Size of the intermediate feed forward layer in each `T5Block`.
         num_layers (`int`, *optional*, defaults to 8):