[UX] Avoid known hosts check (skypilot-org#1614)

* revert error raising as well

* Avoid known hosts check

* address comments

Author: Michaelvll

sky/backends/backend_utils.py

@@ -371,6 +371,12 @@ def _get_generated_config(cls, autogen_comment: str, host_name: str,
             proxy = f'ProxyCommand {proxy_command}'
         else:
             proxy = ''
+        # StrictHostKeyChecking=no skips the host key check for the first
+        # time. UserKnownHostsFile=/dev/null and GlobalKnownHostsFile/dev/null
+        # prevent the host key from being added to the known_hosts file and
+        # always return an empty file for known hosts, making the ssh think
+        # this is a first-time connection, and thus skipping the host key
+        # check.
         codegen = textwrap.dedent(f"""\
             {autogen_comment}
             Host {host_name}
@@ -380,6 +386,8 @@ def _get_generated_config(cls, autogen_comment: str, host_name: str,
               IdentitiesOnly yes
               ForwardAgent yes
               StrictHostKeyChecking no
+              UserKnownHostsFile=/dev/null
+              GlobalKnownHostsFile=/dev/null
               Port 22
               {proxy}
             """.rstrip())