ENH: Add default target layers for gemma2 architecture (#2078)

Google's gemma 2 models have a slightly different architecture than
gemma 1 and thus a different model_type attribute. This PR adds default
target_layer for gemma 2 that correspond to the default target_layer of
gemma 1.

LayerNorm tuning adds one more LN layer.

src/peft/utils/constants.py

@@ -72,6 +72,13 @@ def starcoder_model_postprocess_past_key_value(past_key_values):
     "mistral": ["input_layernorm", "post_attention_layernorm", "norm"],
     "phi": ["input_layernorm", "final_layernorm"],
     "gemma": ["input_layernorm", "post_attention_layernorm", "norm"],
+    "gemma2": [
+        "input_layernorm",
+        "post_attention_layernorm",
+        "pre_feedforward_layernorm",
+        "post_feedforward_layernorm",
+        "norm",
+    ],
     "qwen2": ["post_attention_layernorm"],
 }
 
@@ -107,6 +114,7 @@ def starcoder_model_postprocess_past_key_value(past_key_values):
     "stablelm": ["q_proj", "v_proj"],
     "phi": ["q_proj", "v_proj", "fc1", "fc2"],
     "gemma": ["q_proj", "v_proj"],
+    "gemma2": ["q_proj", "v_proj"],
     "qwen2": ["q_proj", "v_proj"],
 }
 
@@ -133,6 +141,7 @@ def starcoder_model_postprocess_past_key_value(past_key_values):
     "falcon": ["query_key_value", "dense_4h_to_h"],
     "phi": ["q_proj", "v_proj", "fc2"],
     "gemma": ["q_proj", "v_proj", "down_proj"],
+    "gemma2": ["q_proj", "v_proj", "down_proj"],
     "qwen2": ["q_proj", "v_proj", "down_proj"],
 }
 
@@ -159,6 +168,7 @@ def starcoder_model_postprocess_past_key_value(past_key_values):
     "falcon": ["dense_4h_to_h"],
     "phi": ["fc2"],
     "gemma": ["down_proj"],
+    "gemma2": ["down_proj"],
     "qwen2": ["down_proj"],
 }
 
@@ -216,6 +226,7 @@ def starcoder_model_postprocess_past_key_value(past_key_values):
     "stablelm": ["q_proj", "v_proj"],
     "phi": ["q_proj", "v_proj"],
     "gemma": ["q_proj", "v_proj"],
+    "gemma2": ["q_proj", "v_proj"],
     "qwen2": ["q_proj", "v_proj"],
 }
 
@@ -250,6 +261,7 @@ def starcoder_model_postprocess_past_key_value(past_key_values):
     "stablelm": ["q_proj", "v_proj"],
     "phi": ["q_proj", "v_proj", "fc1", "fc2"],
     "gemma": ["q_proj", "v_proj"],
+    "gemma2": ["q_proj", "v_proj"],
     "qwen2": ["q_proj", "v_proj"],
 }