Migrate tests to use BytesIO and temporary paths

[thomas-chauchefoin-tob]

This reduces the number of files created in the local folder. This PR doesn't touch the public API that sometimes expects file paths as arguments, so there will still be a few files here and here the associated tests.

[dguido]

This PR removes the Python 3.14+ compatibility handling that exists in master. The current test_pytorch.py has:

_lacks_torch_jit_support = sys.version_info >= (3, 14)

# In setUp:
if not _lacks_torch_jit_support:
    m = torch.jit.script(model)
    ...

# On tests:
@unittest.skipIf(_lacks_torch_jit_support, "PyTorch 2.9.1 JIT broken with Python 3.14+")
def test_torchscript_wrapper(self):
    ...

The PR removes all of this and unconditionally calls torch.jit.script(model), which will fail on Python 3.14+.

Why it fails: Python 3.14 removed __annotations__ from class instances (PEP 749), and TorchScript relies on it. Calling torch.jit.script() crashes with:

AttributeError: 'Module' object has no attribute '__annotations__'

PyTorch won't fix this because TorchScript is deprecated as of PyTorch 2.9 (replaced by torch.export).

Fix: Preserve the _lacks_torch_jit_support guards when migrating to temp directories:

import sys
_lacks_torch_jit_support = sys.version_info >= (3, 14)

def setUp(self):
    self.tmpdir = tempfile.TemporaryDirectory()
    tmppath = Path(self.tmpdir.name)
    
    model = models.mobilenet_v2()
    self.filename_v1_3 = tmppath / "test_model.pth"
    torch.save(model, self.filename_v1_3)
    
    if not _lacks_torch_jit_support:
        m = torch.jit.script(model)
        self.torchscript_filename = tmppath / "test_model_torchscript.pth"
        torch.jit.save(m, self.torchscript_filename)

And keep the @unittest.skipIf decorators on the TorchScript tests.

Refs:

• torch.jit.script crashes on Python 3.14 in PyTorch 2.9.1 (__annotations__ missing) pytorch/pytorch#167910
• https://docs.pytorch.org/docs/stable/jit.html