add warning to let the user know that the __call__ method is faster than encode + pad for a fast tokenizer (#18693)

* add warning to let the user know that the  method is slower that  for a fast tokenizer

* user warnings

* fix layoutlmv2

* fix layout*

* change warnings into logger.warning

src/transformers/tokenization_utils_base.py

@@ -2821,7 +2821,10 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
         in the batch.
 
         Padding side (left/right) padding token ids are defined at the tokenizer level (with `self.padding_side`,
-        `self.pad_token_id` and `self.pad_token_type_id`)
+        `self.pad_token_id` and `self.pad_token_type_id`).
+
+        Please note that with a fast tokenizer, using the `__call__` method is faster than using a method to encode the
+        text followed by a call to the `pad` method to get a padded encoding.
 
         <Tip>
 
@@ -2871,6 +2874,15 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
             verbose (`bool`, *optional*, defaults to `True`):
                 Whether or not to print more information and warnings.
         """
+        if self.__class__.__name__.endswith("Fast"):
+            if not self.deprecation_warnings.get("Asking-to-pad-a-fast-tokenizer", False):
+                logger.warning_advice(
+                    f"You're using a {self.__class__.__name__} tokenizer. Please note that with a fast tokenizer,"
+                    " using the `__call__` method is faster than using a method to encode the text followed by a call"
+                    " to the `pad` method to get a padded encoding."
+                )
+                self.deprecation_warnings["Asking-to-pad-a-fast-tokenizer"] = True
+
         # If we have a list of dicts, let's convert it in a dict of lists
         # We do this to allow using this method as a collate_fn function in PyTorch Dataloader
         if isinstance(encoded_inputs, (list, tuple)) and isinstance(encoded_inputs[0], Mapping):