Fix LongT5ForConditionalGeneration initialization of lm_head
Author: eranhirs
Pull request: yes

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LongT5ForConditionalGeneration does not work due to a missing initialization of its `lm_head`. Its weight matrices are initialized to zeros and then the logits are all zeros.

Code for reproduction:
```python
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM

tokenizer = AutoTokenizer.from_pretrained("google/long-t5-local-base")
model = AutoModelForSeq2SeqLM.from_pretrained("google/long-t5-local-base")
model.generate(**tokenizer("Generate anything", return_tensors='pt'), max_length=200)
```

This PR contains code copied from [T5PreTrainedModel](https://github.com/huggingface/transformers/blob/v4.37.2/src/transformers/models/t5/modeling_t5.py#L830).

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Comments:
Flax has a different init scheme we should be alright with cross PT FLax tests

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