Fix variable shadowing in pipeline example and typo in BART docs (BERT → BART)

[VanshikaSohal]

What does this PR do?

Fixes two small but impactful bugs in the BART documentation:

1. Variable shadowing bug: In the Pipeline example, the variable
   was named pipeline which shadows the imported pipeline function.
   Renamed to fill_mask_pipeline to avoid confusion and potential errors.

2. Typo in Notes: "BERT uses absolute position embeddings"
   should be "BART uses absolute position embeddings".

Before / After

Before:

pipeline = pipeline(task="fill-mask", ...)
"because BERT uses absolute position embeddings"

After:

fill_mask_pipeline = pipeline(task="fill-mask", ...)
"because BART uses absolute position embeddings"

Related Issue

N/A - documentation fix

Testing

Manually verified the corrected code runs without variable shadowing issues.

[Rocketknight1]

Yep, both good fixes!

[HuggingFaceDocBuilderDev]

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