Fix warning situation: UserWarning: max_length is ignored when padding=True" (#13829)

* Removed wrong warning

* Raise a warning when `max_length` is given with wrong `truncation`

* Update the error message

* Update the warning message
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/tokenization_utils_base.py

@@ -2223,8 +2223,11 @@ class PreTrainedTokenizerBase(SpecialTokensMixin, PushToHubMixin):
         elif padding is not False:
             if padding is True:
                 if verbose:
-                    if max_length is not None:
-                        warnings.warn("`max_length` is ignored when `padding`=`True`.")
+                    if max_length is not None and (truncation is False or truncation == "do_not_truncate"):
+                        warnings.warn(
+                            "`max_length` is ignored when `padding`=`True` and there is no truncation strategy. "
+                            "To pad to max length, use `padding='max_length'`."
+                        )
                     if old_pad_to_max_length is not False:
                         warnings.warn("Though `pad_to_max_length` = `True`, it is ignored because `padding`=`True`.")
                 padding_strategy = PaddingStrategy.LONGEST  # Default to pad to the longest sequence in the batch