output_dir not in ckpt dir

[felipemello1]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

Changelog

As discussed, we wont allow output_dir to be the same or in ckpt_dir. Addresses #2173

Test plan

passes (output_dir != ckpt_dir)

tune run full_finetune_single_device --config llama3_2/3B_full_single_device 

fails (output_dir == ckpt_dir)

tune run full_finetune_single_device --config llama3_2/3B_full_single_device output_dir=/tmp/Llama-3.2-3B-Instruct/

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2181

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✅ No Failures

As of commit daec44b with merge base c6ddf11 ():
💚 Looks good so far! There are no failures yet. 💚

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[ebsmothers]

One more comment, after that and green CI I think this is good to go

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.29630% with 2 lines in your changes missing coverage. Please review.

Project coverage is 66.64%. Comparing base (c0b2cbd) to head (daec44b).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
torchtune/training/checkpointing/_checkpointer.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2181      +/-   ##
==========================================
+ Coverage   66.58%   66.64%   +0.06%     
==========================================
  Files         339      339              
  Lines       20064    20126      +62     
==========================================
+ Hits        13359    13413      +54     
- Misses       6705     6713       +8     

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