[FIX] Fix error max_seq_len == -1

[Hzfengsy]

This one fixes error introduced in #780, where max_seq_len is set to -1 when max_seq_len is not specified in the config file.

[peacepenguin]

Can confirm this fixes mlc-llm rocm with an amd 6800 xt. Can now use the saml_mlc_chat.py no problem.

FYI:
Statistics: prefill: 486.6 tok/s, decode: 72.3 tok/s

Thanks so much @Hzfengsy for this patch!

[junrushao]

Thanks @Hzfengsy for the swift fix!

[masahi]

It's better to assume max_seq_len = 2048 (like before) or use max_sequence_length in config if not specified via cmd line, otherwise split rotary fusion will get disabled.