[profile] make trace viewer respect --path_prefix

[nfelt]

Fixes #2263.

The trace viewer logic was hardcoded to use absolute paths (relative to the hostname), which doesn't work when --path_prefix is set. As far as I can tell there is no particular reason why the trace viewer shouldn't use relative path names like the rest of TensorBoard, and the code that forced the absolute path has been around since the profile plugin was introduced, so I suspect it's just outdated.

Tested by running tensorboard --path_prefix /mypath/ and then putting nginx in front of it as follows:

echo 'events {} http { access_log stdout; error_log stderr; server { listen 8080; server_name localhost; proxy_buffering off; location /mypath/ { proxy_pass http://localhost:6006; }}}' > /tmp/nginx.conf && sudo nginx -g 'daemon off;' -/tmp/nginx.conf

Confirmed that the previously broken trace viewer now loads demo data correctly when accessed via http://localhost:8080/mypath/. I also ran tensorboard without a path prefix and confirmed the trace viewer still loads correctly in that case as well.

[qiuminxu]

It works! I think it is outdated. Thanks for fixing this.