Decouple Post-processing from PointPillars

[kamalrajkannan78]

Ticket

• closes PCC drop in pointpillars, PCC=0.7147460193298709 tt-xla#3986

Problem description

• Pointpillars model fails with PCC=0.7147460193298709
• Pcc drops below 0.99 after sigmoid operation but sigmoid unit test passed with PCC=1
• This suggests the sigmoid op itself is not broken, but is amplifying small numerical differences introduced earlier in the pipeline.
• Since the drop is observed in the post-processing path, that stage can be decoupled from the main model forward.
• For full investigation details, see #3986

What's changed

• Decoupled the PointPillars post-processing path from the main model forward.
• Moved predicted bbox decoding to loader-side post-processing.

Checklist

• verified the changes through local testing

Logs

Before fix

• mar27_pp_cpu_before_fix.log
• mar27_point_pillars_before_fix.log

After fix

• mar27_pp_cpu_after_fix.log
• mar27_point_pillars_xla_after_fix.log