[PluggableDevice] Reduce the log times when plugin is enabled
Hi @ezhulenev Can you please review this PR ? Thank you! Hi @ezhulenev Can you please review this PR ? Thank you!
# Background
TF will print some info in each session as below when plugin is enabled:
```
2022-09-07 09:40:01.716295: I tensorflow/core/grappler/optimizers/custom_graph_optimizer_registry.cc:113] Plugin optimizer for device_type CPU is enabled.
```
These info will be printed **many** times and bother user if there're multiple sessions.

# Solution
Add `call_once` flag for the info, since user won't change the pluggable device after TF is initialized.

Signed-off-by: Lu Teng [teng.lu@intel.com](mailto:teng.lu@intel.com)