Allow users assign specific index for PAD or UNK in Vocabulary

**Is your feature request related to a problem? Please describe.**
Currently, when building vocabulary, user can only say need_pad, then Vocabulary assign a fix index to ['PAD'], which is 0 or -1 (if 'one-hot'). However, in some cases, user may want to use a specific index for ['PAD'].

**Describe the solution you'd like**
Make pad_value and unk_value configurable.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.

Start by locating the Vocabulary implementation and its existing handling of need_pad, the PAD index, one-hot mode, and UNK. Review related tests if present, then define the configurable pad_value and unk_value behavior and verify that callers can assign specific indices without breaking current defaults.

Written by the indexing model from the issue text.