Remove DataPack.text property

[hepengfe]

Describe the bug
With the introduction of Payload with modalities, we have multiple payloads in the datapack. Therefore, it's not good to continue to use DataPack.text which could be ambiguous in which text payload it refers to. In the past, DataPack.text refers to the single text in the DataPack, and for backward compatibility we let it be the text in the first TextPayload.

To Reproduce
Steps to reproduce the behavior:

1. Prepare data ...
2. Run command '....'
3. See error

Expected behavior

• DataPack.text is removed
• All code dependent on it will be changed to use DataPack.get_payload_data_at(Modality.Text, 0)

Screenshots
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Environment (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]
• Python and Package verions: [e.g. Python version, Pytorch version]

Additional context
Add any other context about the problem here.

[hunterhector]

No, don't change existing APIs. The payload API itself is much longer and non-user friendly

[hepengfe]

I removed DataPack.audio already in #828. Should I add it back?

[hunterhector]

hmm, sorry I didn't notice that one. Yes, let's add it back. Now is not the time to change the interfaces. Instead we should make sure all old interfaces work.

[hunterhector]

I removed DataPack.audio already in #828. Should I add it back?

@hepengfe let's add this back as an issue to track it.