Add warning in `generate` & `device_map=auto` & half precision models (#19468)

* fix device mismatch

* make fixup

* added slow tests

- added slow tests on `bnb` models to make sure generate works correctly

* replace with `self.device`

* revert force device assign

* Update src/transformers/generation_utils.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* set the warning in `generate` instead of `sample`
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/generation_utils.py

@@ -1349,6 +1349,17 @@ class GenerationMixin:
                 "Diverse beam search cannot be used in sampling mode. Make sure that `do_sample` is set to `False`."
             )
 
+        if self.device.type != input_ids.device.type:
+            warnings.warn(
+                "You are calling .generate() with the `input_ids` being on a device type different"
+                f" than your model's device. `input_ids` is on {input_ids.device.type}, whereas the model"
+                f" is on {self.device.type}. You may experience unexpected behaviors or slower generation."
+                " Please make sure that you have put `input_ids` to the"
+                f" correct device by calling for example input_ids = input_ids.to('{self.device.type}') before"
+                " running `.generate()`.",
+                UserWarning,
+            )
+
         # 7. prepare distribution pre_processing samplers
         logits_processor = self._get_logits_processor(
             repetition_penalty=repetition_penalty,
@@ -1976,7 +1987,6 @@ class GenerationMixin:
         >>> tokenizer.batch_decode(outputs, skip_special_tokens=True)
         ['Today is a beautiful day, and a wonderful day.\n\nI was lucky enough to meet the']
         ```"""
-
         # init values
         logits_processor = logits_processor if logits_processor is not None else LogitsProcessorList()
         stopping_criteria = stopping_criteria if stopping_criteria is not None else StoppingCriteriaList()