chore(optim): wrap torch.autograd.grad() with torch.enable_grad() context

[XuehaiPan]

Description

Describe your changes in detail.

Explicitly wrap torch.autograd.grad() with torch.enable_grad() context. Users can use inference-only mode for hyper gradient with:

meta_optim = torchopt.MetaAdam(model, lr=0.1)

loss = compute_loss(model, batch)
with torch.no_grad():
    meta_optim.step(loss)

Motivation and Context

Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.
You can use the syntax close #15213 if this solves the issue #15213

• I have raised an issue to propose this change (required for new features and bug fixes)

#218 (comment)

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)
• Example (update in the folder of example)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.
• I have reformatted the code using make format. (required)
• I have checked the code using make lint. (required)
• I have ensured make test pass. (required)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.69%. Comparing base (b3f570c) to head (5f9b918).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #220   +/-   ##
=======================================
  Coverage   93.69%   93.69%           
=======================================
  Files          83       83           
  Lines        2963     2965    +2     
=======================================
+ Hits         2776     2778    +2     
  Misses        187      187           

Flag	Coverage Δ
unittests	93.69% <100.00%> (+<0.01%)	⬆️

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