Add options to split temporary artifacts in per-stage subdirectories

[PhilippvK]

A MLonMCU "run" typically consists of the following stages:

• LOAD: Process the model in the frotend to produce e.g. a .tflite file
• TUNE: Generate tuning records (Optional)
• BUILD: Run the chosen backend to generate (wrapper) code for the model
• COMPILE: Compile the target software using the generated code
• RUN: Run the resulting ELF on the defined target platform.
• (POSTPROCESS: unimplemented)

Currently the intermediate artifacts are dumped in a single directory $MLONMCU_HOME/temp/sessions/$SID/runs/$RID/ (with SID being the session ID and RID the run ID)

I would like to add an config option artifacts_per_stage=1/0 which can be defined on the command line of the the environments.yml to instead use subdiretories for every stage i.e. $MLONMCU_HOME/temp/sessions/$SID/runs/$RID/{load,build,compile,run,postprocess}

[PhilippvK]

This is partly implemented. Currently it generates a stages directory with subdirectories for each used stage. Currently the stage numbers (e.g. 0,2,3,4) are used instead of their names. The platform directory stays in the top level run directory as it is used by several stages (i.e. (Micro)TVM is used for TUNE, BUILD, COMPILE, RUN)