Make new_system_prompt of InputOutputToMessages configurable in instruct_dataset builder

[mreso]

Context

What is the purpose of this PR? Is it to

• add a new feature

Please link to any issues this PR addresses.

Changelog

What are the changes made in this PR?
Let's users create a custom instruct dataset with a custom system prompt without creating a custom dataset builder

Users can config their custom instruct dataset like:

# Dataset and Sampler
dataset:
  _component_: your.custom.folder.instruct_dataset
  source: "Samsung/samsum"
  train_on_input: False
  column_map:
    input: dialogue
    output: summary
  new_system_prompt: "Summarize the given text into a single sentence" # <<<----
  split: train

which previously required a custom builder function

Test plan

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UX

If your function changed a public API, please add a dummy example of what the user experience will look like when calling it.
Example of docstring: https://github.com/pytorch/torchtune/blob/6a7951f1cdd0b56a9746ef5935106989415f50e3/torchtune/modules/vision_transformer.py#L285
Example in our docs: https://pytorch.org/torchtune/main/tutorials/qat_finetune.html#applying-qat-to-llama3-models

• I did not change any public API;
• I have added an example to docs or docstrings;

[pytorch-bot]

🔗 Helpful Links

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As of commit 2303be0 with merge base 4fba6cd ():
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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.01%. Comparing base (ec21546) to head (2303be0).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1465      +/-   ##
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+ Coverage   70.01%   72.01%   +2.00%     
==========================================
  Files         268      272       +4     
  Lines       12937    13059     +122     
==========================================
+ Hits         9058     9405     +347     
+ Misses       3879     3654     -225     

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