Update troubleshoot with more content (#16243)

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docs/source/troubleshooting.mdx

@@ -120,4 +120,57 @@ Another option is to get a better traceback from the GPU. Add the following envi
 >>> import os
 
 >>> os.environ["CUDA_LAUNCH_BLOCKING"] = "1"
-```
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+```
+
+## Incorrect output when padding tokens aren't masked
+
+In some cases, the output `hidden_state` may be incorrect if the `input_ids` include padding tokens. To demonstrate, load a model and tokenizer. You can access a model's `pad_token_id` to see its value. The `pad_token_id` may be `None` for some models, but you can always manually set it.
+
+```py
+>>> from transformers import AutoModelForSequenceClassification
+>>> import torch
+
+>>> model = AutoModelForSequenceClassification.from_pretrained("bert-base-uncased")
+>>> model.config.pad_token_id
+0
+```
+
+The following example shows the output without masking the padding tokens:
+
+```py
+>>> input_ids = torch.tensor([[7592, 2057, 2097, 2393, 9611, 2115], [7592, 0, 0, 0, 0, 0]])
+>>> output = model(input_ids)
+>>> print(output.logits)
+tensor([[ 0.0082, -0.2307],
+        [ 0.1317, -0.1683]], grad_fn=<AddmmBackward0>)
+```
+
+Here is the actual output of the second sequence:
+
+```py
+>>> input_ids = torch.tensor([[7592]])
+>>> output = model(input_ids)
+>>> print(output.logits)
+tensor([[-0.1008, -0.4061]], grad_fn=<AddmmBackward0>)
+```
+
+Most of the time, you should provide an `attention_mask` to your model to ignore the padding tokens to avoid this silent error. Now the output of the second sequence matches its actual output:
+
+<Tip>
+
+By default, the tokenizer creates an `attention_mask` for you based on your specific tokenizer's defaults.
+
+</Tip>
+
+```py
+>>> attention_mask = torch.tensor([[1, 1, 1, 1, 1, 1], [1, 0, 0, 0, 0, 0]])
+>>> output = model(input_ids, attention_mask=attention_mask)
+>>> print(output.logits)
+tensor([[ 0.0082, -0.2307],
+        [-0.1008, -0.4061]], grad_fn=<AddmmBackward0>)
+```
+
+🤗 Transformers doesn't automatically create an `attention_mask` to mask a padding token if it is provided because:
+
+- Some models don't have a padding token.
+- For some use-cases, users want a model to attend to a padding token.
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