add PT_XLA_DEBUG_LEVEL

[JackCaoG]

PT_XLA_DEBUG_LEVEL=1 will not output the executation frame analysis. Also update the post-compilation analysis to GB instead of MB. sample output

Compilation Analysis: ================================================================================
Compilation Analysis: Compilation Cause
Compilation Analysis:   mark_step in parallel loader at step end
Compilation Analysis: Graph Info: 
Compilation Analysis:   Graph Hash: c74c3b91b855b2b123f833b0d5f86943
Compilation Analysis:   Number of Graph Inputs: 35
Compilation Analysis:   Number of Graph Outputs: 107
Compilation Analysis: Python Frame Triggered Execution: 
Compilation Analysis:   mark_step (/workspaces/dk3/pytorch/xla/torch_xla/core/xla_model.py:1055)
Compilation Analysis:   next (/workspaces/dk3/pytorch/xla/torch_xla/distributed/parallel_loader.py:44)
Compilation Analysis:   __next__ (/workspaces/dk3/pytorch/xla/torch_xla/distributed/parallel_loader.py:32)
Compilation Analysis:   train_loop_fn (/workspaces/dk3/pytorch/xla/examples/train_decoder_only_base.py:48)
Compilation Analysis:   start_training (/workspaces/dk3/pytorch/xla/examples/train_decoder_only_base.py:65)
Compilation Analysis:   <module> (/workspaces/dk3/pytorch/xla/examples/train_decoder_only_base.py:73)
Compilation Analysis: --------------------------------------------------------------------------------
Compilation Analysis: ================================================================================

Post Compilation Analysis: ================================================================================
Post Compilation Analysis: Graph input size: 1.548000 GB
Post Compilation Analysis: Graph output size: 7.922460 GB
Post Compilation Analysis: Aliased Input size: 1.547871 GB
Post Compilation Analysis: Intermediate tensor size: 12.124478 GB
Post Compilation Analysis: Compiled program size: 0.028210 GB
Post Compilation Analysis: --------------------------------------------------------------------------------
Post Compilation Analysis: ================================================================================
epoch: 1, step: 0, loss: 7.349868297576904, rate: 2.489864196404525

[JackCaoG]

This should be ready for review.

[will-cromar]

Thanks!