Feat: form parsing placeholders

[MillCheck]

Allows introduction of form extraction in the future - sets up the FormKeysValues element & format, puts in an empty function call in the partition_pdf_or_image pipeline.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: unstructured/documents/elements.py

Function	Unhandled Issue
to_dict	**ValueError: operands could not be broadcast together with shapes (768,) (0,) ** ... Event Count: 8

---

_{Did you find this useful? React with a 👍 or 👎}

[MillCheck]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: unstructured/documents/elements.py

Function Unhandled Issue
to_dict **ValueError: operands could not be broadcast together with shapes (768,) (0,) ** ...
Event Count: 8
Did you find this useful? React with a 👍 or 👎

I did not touch that and I refuse to use this PR to fix that.

[MthwRobinson]

Looks good! Couple small coments.

[MthwRobinson]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:
📄 File: unstructured/documents/elements.py
Function Unhandled Issue
to_dict **ValueError: operands could not be broadcast together with shapes (768,) (0,) ** ...
Event Count: 8
Did you find this useful? React with a 👍 or 👎

I did not touch that and I refuse to use this PR to fix that.

Concur, not sure why that alert appeared.

[MillCheck]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:
📄 File: unstructured/documents/elements.py
Function Unhandled Issue
to_dict **ValueError: operands could not be broadcast together with shapes (768,) (0,) ** ...
Event Count: 8
Did you find this useful? React with a 👍 or 👎

I did not touch that and I refuse to use this PR to fix that.

Concur, not sure why that alert appeared.

The error is rather sensible - many numpy/pandas/similar objects overwrite the == operator into a broadcasting one, one of the previous pull requests likely allowed such situation to happen. Now any change to elements.py will get this notification ;)

[MthwRobinson]

LGTM pending a minor changelog nit!