Fix several issues in tegra observer

[loostrum]

I ran into the following exception on a Jetson:

Tuning ...
Exception ignored while calling deallocator <function tegra.__del__ at 0xffff9ad38670>:
Traceback (most recent call last):
  File "/home/powersensor/kt_demo/kernel_tuner/kernel_tuner/observers/tegra.py", line 171, in __del__
    if self.has_changed_clocks:
AttributeError: 'tegra' object has no attribute 'has_changed_clocks'
Traceback (most recent call last):
...

The actual error is that I didn't specify the required input arguments to the Tegraclass. Apparently such a failure in __init__ means that any attributes defined in __init__ do not exist, but __del__ is still called at cleanup resulting in the exception. This PR fixes this by first checking whether the attribute exists before checking its value.

[loostrum]

Putting this back to draft because I actually found a few more issues in the Tegra observer, I will have a look at those at well.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[loostrum]

Other issues fixed:

• get_tegra_gr_clocks failed because it initialized the Tegra class without the required arguments
• The GPU temperature sensor name apparently varies between Jetson variants; I've added the name I see on my Jetson Orin Nano
• The raw frequency to int conversion failed for the length-1 case

With these fixes, the tegra observer is working as expected again.

[benvanwerkhoven]

Thanks @loostrum !