Generate: deprecate old src imports (#28607)

src/transformers/generation_flax_utils.py

@@ -23,6 +23,6 @@ class FlaxGenerationMixin(FlaxGenerationMixin):
     # warning at import time
     warnings.warn(
         "Importing `FlaxGenerationMixin` from `src/transformers/generation_flax_utils.py` is deprecated and will "
-        "be removed in Transformers v5. Import as `from transformers import FlaxGenerationMixin` instead.",
+        "be removed in Transformers v4.40. Import as `from transformers import FlaxGenerationMixin` instead.",
         FutureWarning,
     )

src/transformers/generation_tf_utils.py

@@ -23,6 +23,6 @@ class TFGenerationMixin(TFGenerationMixin):
     # warning at import time
     warnings.warn(
         "Importing `TFGenerationMixin` from `src/transformers/generation_tf_utils.py` is deprecated and will "
-        "be removed in Transformers v5. Import as `from transformers import TFGenerationMixin` instead.",
+        "be removed in Transformers v4.40. Import as `from transformers import TFGenerationMixin` instead.",
         FutureWarning,
     )

src/transformers/generation_utils.py

@@ -23,6 +23,6 @@ class GenerationMixin(GenerationMixin):
     # warning at import time
     warnings.warn(
         "Importing `GenerationMixin` from `src/transformers/generation_utils.py` is deprecated and will "
-        "be removed in Transformers v5. Import as `from transformers import GenerationMixin` instead.",
+        "be removed in Transformers v4.40. Import as `from transformers import GenerationMixin` instead.",
         FutureWarning,
     )