Initialize local CPU server if remote server not available

[kpedro88]

Initializing a local CPU server when the requested remote server is not available can serve as a fallback option. Open questions:

1. At what stage should checking availability and initializing the local server take place? (python config, C++ module initialization, ....)
2. How to control # threads used by the local server? see Control number of threads used by CPU server triton-inference-server/server#2018

If a local CPU server is used, SonicTriton clients should be forced to use Sync mode, to prevent contention between CMSSW and the local server (otherwise they would both try to use the same threads).

Assigned to: @kpedro88

[kpedro88]

See cms-sw/cmssw@master...kpedro88:TritonService for a functioning draft version of TritonService with automatic fallback support. Notes:

• currently based on CMSSW_11_2_0_pre9
• still potentially a work in progress, so the branch may be updated at any moment - don't get attached
• all other ongoing SONIC-related PRs and branches will have to be rebased on this once it is merged (some API changes), but let's not worry about that yet

[kpedro88]

draft PR now submitted: cms-sw/cmssw#32576

[kpedro88]

minor followup: cms-sw/cmssw#32861