Fix (examples/llm): Fix infinite loop in LLM entrypoint with WikiText2

[pablomlago]

Fix for issue #1036. When fuse_sequences is False, and the seqlen is greater than 842 (maximum sequence length in WikiText2), the condition in line 131 of optimum/amd/brevitas/data_utils.py always evaluates to True, resulting in an infinite loop (see commit d0d06070225804dc6fecde00911ae38b53080123 in optimum-amd).

Consequently, we opted for enabling passing fuse_sequences as an argument and warning the user, when fuse_sequences=False, than an overly large seqlen can result in the infinite loop indicated in issue #1036.

[pablomlago]

The distribution of the sequence lengths in WikiText2, when fuse_sequences is set to False, is shown in the following picture:

Note that, for the default value seqlen=2048, the infinite loop is entered, as the maximum sequence length is 842.

[Giuseppe5]

Can you update the readme?
Generate the usage description with
python main.py -h
And copy and past it in the readme