Fixes warning when initializing prompt encoder (#716)

Right now, when the user initializes a prompt encoder with MLP, they get
a warning that a certain argument is ignored, and there is no possible
value for the argument that would stop the warning. Usually, warnings
are for issues that something is (probably) going wrong, but here,
everything is going as expected. Therefore, by default, I would not give
this warning, thus avoiding users getting confused.

However, I would still give the warning if the user set the argument for
encoder_num_layers explicitly to a different value. In that case, they
expect the change to make a difference, but since the argument is
ignored, their expectation is not met, which warrants a warning.

src/peft/tuners/p_tuning.py

@@ -143,9 +143,12 @@ def __init__(self, config):
                 )
 
             elif self.encoder_type == PromptEncoderReparameterizationType.MLP:
-                warnings.warn(
-                    f"for {self.encoder_type}, the `encoder_num_layers` is ignored. Exactly 2 MLP layers are used."
-                )
+                encoder_num_layers_default = PromptEncoderConfig.encoder_num_layers
+                if config.encoder_num_layers != encoder_num_layers_default:
+                    warnings.warn(
+                        f"for {self.encoder_type}, the argument `encoder_num_layers` is ignored. "
+                        f"Exactly {encoder_num_layers_default} MLP layers are used."
+                    )
                 layers = [
                     torch.nn.Linear(self.input_size, self.hidden_size),
                     torch.nn.ReLU(),

tests/test_config.py

@@ -17,13 +17,17 @@
 import pickle
 import tempfile
 import unittest
+import warnings
+
+import pytest
 
 from peft import (
     AdaptionPromptConfig,
     IA3Config,
     LoraConfig,
     PeftConfig,
     PrefixTuningConfig,
+    PromptEncoder,
     PromptEncoderConfig,
     PromptTuningConfig,
 )
@@ -156,3 +160,25 @@ def test_config_pickle_roundtrip(self):
             config = config_class()
             copied = pickle.loads(pickle.dumps(config))
             self.assertEqual(config.to_dict(), copied.to_dict())
+
+    def test_prompt_encoder_warning_num_layers(self):
+        # This test checks that if a prompt encoder config is created with an argument that is ignored, there should be
+        # warning. However, there should be no warning if the default value is used.
+        kwargs = {
+            "num_virtual_tokens": 20,
+            "num_transformer_submodules": 1,
+            "token_dim": 768,
+            "encoder_hidden_size": 768,
+        }
+
+        # there should be no warning with just default argument for encoder_num_layer
+        config = PromptEncoderConfig(**kwargs)
+        with warnings.catch_warnings():
+            PromptEncoder(config)
+
+        # when changing encoder_num_layer, there should be a warning for MLP since that value is not used
+        config = PromptEncoderConfig(encoder_num_layers=123, **kwargs)
+        with pytest.warns(UserWarning) as record:
+            PromptEncoder(config)
+        expected_msg = "for MLP, the argument `encoder_num_layers` is ignored. Exactly 2 MLP layers are used."
+        assert str(record.list[0].message) == expected_msg