[Docker] docker mode fails when nvidia-smi is not installed

[romilbhardwaj]

After #411, LocalDockerBackend is broken for users who don't have nvidia-smi installed. Report from @gmittal:

sky ❯ sky launch examples/minimal.yaml --docker
Task from YAML spec: examples/minimal.yaml
Traceback (most recent call last):
  File "/Users/gautam/miniconda3/envs/sky/bin/sky", line 33, in <module>
    sys.exit(load_entry_point('sky', 'console_scripts', 'sky')())
  File "/Users/gautam/miniconda3/envs/sky/lib/python3.8/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/Users/gautam/miniconda3/envs/sky/lib/python3.8/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/Users/gautam/miniconda3/envs/sky/lib/python3.8/site-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/gautam/miniconda3/envs/sky/lib/python3.8/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/gautam/miniconda3/envs/sky/lib/python3.8/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/Users/gautam/Desktop/workbench/sky/sky/cli.py", line 521, in launch
    backend = backends.LocalDockerBackend()
  File "/Users/gautam/Desktop/workbench/sky/sky/backends/local_docker_backend.py", line 108, in __init__
    self._use_gpu = backend_utils.check_local_gpus() if use_gpu == 'auto' \
  File "/Users/gautam/Desktop/workbench/sky/sky/backends/backend_utils.py", line 752, in check_local_gpus
    execution_check = subprocess.run(['nvidia-smi'],
  File "/Users/gautam/miniconda3/envs/sky/lib/python3.8/subprocess.py", line 493, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/Users/gautam/miniconda3/envs/sky/lib/python3.8/subprocess.py", line 858, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Users/gautam/miniconda3/envs/sky/lib/python3.8/subprocess.py", line 1704, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'nvidia-smi'

this line is likely culprit:
https://github.com/sky-proj/sky/blob/d19a6fd2a5b7f7da3d1b3bfa2c2fb40c9d90b27f/sky/backends/backend_utils.py#L751

It should just check:
if is_installed:

[WoosukKwon]

Oh I'm really sorry. Thanks for reporting this problem @gmittal @romilbhardwaj.