Add trace dataflow correctness tests

## 🚀 Feature

As one of the core principles of Thunder is having a dataflow from input to output, there should be some tests to verify that the transformations respect that.

Simple example of test might be to run dce on the trace we wish to check. If the trace is not runnable anymore after dce, then the dataflow was not correct.

cc @mruberry

The issue names no files or existing tests; start by locating the trace transformation tests and the DCE entry point. Define the checks around whether transformed traces remain runnable after DCE, and consider the work done when tests cover dataflow correctness across the relevant transformations.

Written by the indexing model from the issue text.