Fix incorrect cache allocation with multi-query (#2203)

We wouldn't allocate any memory in multi-query (1 KV head). Fixes
Starcoder et al.

server/text_generation_server/models/flash_causal_lm.py

@@ -912,7 +912,12 @@ class FlashCausalLM(Model):
                     break
             if num_kv_heads is None:
                 raise ValueError("Cannot get the number of key/value heads")
-        self.num_kv_heads = num_kv_heads // self.process_group.size()
+        self.num_kv_heads = (
+            num_kv_heads // self.process_group.size()
+            if num_kv_heads > 1
+            else num_kv_heads
+        )
+        assert self.num_kv_heads > 0
         self.head_size = config.hidden_size // config.num_attention_heads
 
         self.cuda_graphs = {}