Fix for stochastic depth decay rule in the TimeSformer implementation
Author: atawari
Pull request: yes

Fixing typo to correct the stochastic depth decay rule

# What does this PR do?

Implementation of `drop_path` module in TimesformerLayer uses a constant config.drop_path as oppose to stochastic drop path as expected - see comments [here](https://github.com/huggingface/transformers/blob/main/src/transformers/models/timesformer/modeling_timesformer.py#L305) and authors implementation [here](https://github.com/facebookresearch/TimeSformer/blob/a5ef29a7b7264baff199a30b3306ac27de901133/timesformer/models/vit.py#L205)

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Comments:
tagging @amyeroberts as the change is in the vision module.

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