use split/squeeze instead of slice for performance (#409)

* use split/squeeze instead of slice for performance

GPU may not have perf difference but HPU perf improves with this

* add copyrights

megatron/model/transformer.py

@@ -1,3 +1,4 @@
+# Copyright (C) 2024 Habana Labs, Ltd. an Intel Company.
 # Copyright (c) 2023, NVIDIA CORPORATION. All rights reserved.
 
 """Transformer."""
@@ -660,9 +661,10 @@ def repeat_kv(self, hidden_states, n_rep):
                                      head_dim)
 
     def split_tensor(self, mixed_x_layer):
-        query_layer = mixed_x_layer[:, :, :, :-2, :].reshape(mixed_x_layer.shape[:2] + (-1, self.hidden_size_per_attention_head))
-        key_layer = mixed_x_layer[:, :, :, -2, :]
-        value_layer = mixed_x_layer[:, :, :, -1, :]
+        query_layer, key_layer, value_layer = torch.split(mixed_x_layer, [self.num_key_value_groups, 1, 1], dim=-2)
+        query_layer = query_layer.reshape(mixed_x_layer.shape[:2] + (self.num_attention_heads_per_partition, self.hidden_size_per_attention_head))
+        key_layer = torch.squeeze(key_layer, -2)
+        value_layer = torch.squeeze(value_layer, -2)
 
         return query_layer, key_layer, value_layer