[V1][Spec Dec Bug Fix] Respect Spec Dec Method Specification

[luyuzhe111]

Currently, we try to automatically detect the speculative decoding method. For EAGLE, it detects via the draft model checkpoint folder. However, if the folder name does not have the eagle- substring, the current logic will make the method draft_model even if one has directly specify the method as eagle, which leads to subsequent errors. This fix avoids this confusion.

@LiuXiaoxuanPKU

[github-actions]

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[LiuXiaoxuanPKU]

LGTM