[Enhancement]Improve the logic for find free port in distributed training

[imabackstabber]

Motivation

deal with some cases mentioned in issue #1865

Modification

1.add print_log to log the updated free port to inform users since it is different from the expected port 2.when the port set by users is not free,find a free port and log the free port to inform users.

Checklist

Before PR:

• [x] I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• [x] Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• [x] Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• [x] New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• [x] The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• [x] If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• [x] CLA has been signed and all committers have signed the CLA in this PR.

[zhouzaida]

pytest tests/test_runner/test_dist_utils.py::test_init_dist -s -v

FAILED tests/test_runner/test_dist_utils.py::test_init_dist - socket.gaierror: [Errno -2] Name or service not known

[imabackstabber]

pytest tests/test_runner/test_dist_utils.py::test_init_dist -s -v

FAILED tests/test_runner/test_dist_utils.py::test_init_dist - socket.gaierror: [Errno -2] Name or service not known

This is probably caused by /etc/hosts not properly set up, see https://stackoverflow.com/questions/38254394/name-or-service-not-known for more details.