[fix] properly propagate swapped proxies in TraceSubstitutionProcessor

[ali-alshaar7]

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What does this PR do?

fixes 1641

the swap_map used to replace proxies following substitution transforms did not properly propagate new proxies to other bsyms.

PR review

Also adds a test for a simple buffer casting/quantization transform which used to throw key errors.

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[ali-alshaar7]

Some context on the failures we're running into here:

rematerialization transforms occur at some point after the fw/bw traces have been created, where transforms have been applied onto each respective transform already.

Remat then continues to assume that these traces are still explicitly linked, which used to be true when tracesubstitution didn't properly propagate new proxies, but now that it does, the remat runs into issues.

[t-vi]

Thank you @ali-alshaar7