Fix absolute path in path join call
# What does this PR do ?

This fixes a regression introduced in 39aff5ca3d52247e136a86429d567255c02a5d44 when joining paths was changed from string concatenation to `os.path.join`. At the moment the call disregards the first part (`frame_pred_dir`) and always returns a path starting with `/overlap_smoothing_output`.

**Collection**:  asr

# Changelog 

- Remove leading slash in argument to `os.path.join`

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- [x] Bugfix
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# Additional Information

