[WIP] Add support for Mistral-Nemo by supporting head_dim through config (#2254)

* Support passing head_dim through config

* Using `head_dim` as a fallback is necessary since it's a non standard
key in mistralConfig (as defined in transformers).

* Shorter diff.

---------
Co-authored-by: Nicolas Patry <patry.nicolas@protonmail.com>

server/text_generation_server/models/custom_modeling/flash_mistral_modeling.py

@@ -149,15 +149,14 @@ class MistralAttention(torch.nn.Module):
             bias=False,
         )
 
-        head_size = config.hidden_size // config.num_attention_heads
         self.query_key_value = TensorParallelMultiAdapterLinear.load(
             query_key_value,
             layer_id,
             ["q_proj", "k_proj", "v_proj"],
             sizes=[
-                head_size * config.num_attention_heads,
-                head_size * config.num_key_value_heads,
-                head_size * config.num_key_value_heads,
+                self.head_size * config.num_attention_heads,
+                self.head_size * config.num_key_value_heads,
+                self.head_size * config.num_key_value_heads,
             ],
             process_group=weights.process_group,
         )