torchvision.io: don't warn about missing image libraries unless necessary

🐛 Describe the bug

If no image libraries are installed, the following will warn:

>>> import torchvision
UserWarning: Failed to load image Python extension: 

I propose we only warn when importing torchvision.io or when using a specific function that requires one of these image libraries.

Related to #7151

Versions

Collecting environment information...
Traceback (most recent call last):
  File "/Users/Adam/Downloads/collect_env.py", line 624, in <module>
    main()
  File "/Users/Adam/Downloads/collect_env.py", line 607, in main
    output = get_pretty_env_info()
             ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/Adam/Downloads/collect_env.py", line 602, in get_pretty_env_info
    return pretty_str(get_env_info())
                      ^^^^^^^^^^^^^^
  File "/Users/Adam/Downloads/collect_env.py", line 438, in get_env_info
    pip_version, pip_list_output = get_pip_packages(run_lambda)
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/Adam/Downloads/collect_env.py", line 410, in get_pip_packages
    out = run_with_pip([sys.executable, '-mpip'])
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/Adam/Downloads/collect_env.py", line 405, in run_with_pip
    for line in out.splitlines()
                ^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'splitlines'

Reproduce the warning by importing torchvision with no image libraries installed, then inspect the import paths for torchvision and torchvision.io. Verify which image-dependent functions require those libraries. Done means importing torchvision is quiet while importing torchvision.io or calling a dependent function still warns when appropriate.