Bug Fix: Update the port scanning logic to properly skip ports in use (#6560)

## Motivation for features / changes
#6552

## Technical description of changes
The port scanning logic originally added in #1851 does not seem to work
anymore. WerkzeugServer no longer raises an Error when attempting to
bind to a port which is in used and instead handles the issue silently.

The current Werkzeug version is 2.3.7 but I had 2.3.4 installed when I
first verified this. I tried installing 2.2.0 and the issues was not
present so I assume it was introduced in 2.3.0.
https://pypi.org/project/Werkzeug

## Detailed steps to verify changes work correctly (as executed by you)
1) Start TensorBoard, note the port it is bound to (it should be 6006)
2) In a separate terminal start TensorBoard again.
3) TensorBoard should successfully bind to a different port (this time
6007)

Author: rileyajones

tensorboard/program.py

@@ -205,7 +205,7 @@ def configure(self, argv=("",), **kwargs):
             # any positional arguments to `serve`.
             serve_parser = serve_subparser
 
-        for (name, subcommand) in self.subcommands.items():
+        for name, subcommand in self.subcommands.items():
             subparser = subparsers.add_parser(
                 name,
                 help=subcommand.help(),
@@ -698,7 +698,17 @@ def __init__(self, wsgi_app, flags):
         self._url = None  # Will be set by get_url() below
 
         self._fix_werkzeug_logging()
+
+        def is_port_in_use(port):
+            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+                return s.connect_ex(("localhost", port)) == 0
+
         try:
+            if is_port_in_use(port):
+                raise TensorBoardPortInUseError(
+                    "TensorBoard could not bind to port %d, it was already in use"
+                    % port
+                )
             super().__init__(host, port, wsgi_app, _WSGIRequestHandler)
         except socket.error as e:
             if hasattr(errno, "EACCES") and e.errno == errno.EACCES: