Update gemma-2 kvcache constructor and fix mask type check.

Author: mirceamironenco

torchtune/models/gemma2/_attention.py

@@ -149,7 +149,7 @@ def setup_cache(
             self.kv_cache = KVCache(
                 batch_size=batch_size,
                 max_seq_len=max_seq_len,
-                num_heads=self.num_heads,
+                num_kv_heads=self.num_heads,
                 head_dim=self.head_dim,
                 dtype=dtype,
             )
@@ -211,9 +211,9 @@ def forward(
             - h_d: head dim
         """
         #  until flex attention implementation exists, we do not accept block masks
-        if (mask is not None) and (type(mask) != torch.Tensor()):
+        if mask is not None and (not isinstance(mask, torch.Tensor)):
             raise NotImplementedError(
-                "Block masks are not implemeted yet, use packed=False"
+                "Block masks are not implemeted yet, use packed=False."
             )
 
         # x has shape [b, s_x, d]