Fixing watermark. (#851)

# What does this PR do?

Fixes #843 
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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or...

server/text_generation_server/utils/watermark.py

@@ -19,8 +19,8 @@ import torch
 from transformers import LogitsProcessor
 from typing import List, Union
 
-GAMMA = os.getenv("WATERMARK_GAMMA", 0.5)
-DELTA = os.getenv("WATERMARK_DELTA", 2.0)
+GAMMA = float(os.getenv("WATERMARK_GAMMA", 0.5))
+DELTA = float(os.getenv("WATERMARK_DELTA", 2.0))
 
 
 class WatermarkLogitsProcessor(LogitsProcessor):