feat: add Metharme prompt strategy (#446)

* Add Metharme tokenizing strategy

This strategy accounts for how the Metharme JSONLs are formatted as well as adds duplicated EOS tokens which can help trim model output length.
I haven't gotten the chance to test this yet, and probably won't have the chance for quite a bit, so I'm committing this now.

* Redo Metharme tokenizing strategy

lol

* fix: oops

* Rearrange a conditional

* chore: reformat code in accordance with linter

* chore: Make lint not freak out

* chore: fix lint

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Co-authored-by: NanoCode012 <kevinvong@rocketmail.com>

README.md

@@ -257,6 +257,10 @@ Have dataset(s) in one of the following format (JSONL recommended):
   ```json
   {"conversations": [{"role": "...", "value": "..."}]}
   ```
+- `metharme`: instruction, adds additional eos tokens
+  ```json
+  {"prompt": "...", "generation": "..."}
+  ```
 - `sharegpt_simple.load_role`: conversations where `role` is used instead of `from`
   ```json
   {"conversations": [{"role": "...", "value": "..."}]}