Updated docs of forward in Idefics2ForConditionalGeneration with correct ignore_index value

[zafstojano]

What does this PR do?

The docs in the forward method of the Idefics2ForConditionalGeneration incorrectly states that -100 is the value in the labels variable for which the CrossEntropyLoss will ignore the appropriate indices:

transformers/src/transformers/models/idefics2/modeling_idefics2.py

Lines 1777 to 1780 in 277db23

	labels (`torch.LongTensor` of shape `(batch_size, sequence_length)`, *optional*):
	Labels for computing the masked language modeling loss. Indices should either be in `[0, ...,
	config.vocab_size]` or -100 (see `input_ids` docstring). Tokens with indices set to `-100` are ignored
	(masked), the loss is only computed for the tokens with labels in `[0, ..., config.vocab_size]`.

However, it can be seen in this line that in fact the model uses the image_token_id as an ignore index:

transformers/src/transformers/models/idefics2/modeling_idefics2.py

Line 1860 in 277db23

loss_fct = CrossEntropyLoss(ignore_index=self.image_token_id)

This PR updates the docs with the correct information.

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Ping @amyeroberts

[amyeroberts]

Thanks for fixing!