[autodiff] Skip store forwarding to keep the GlobalLoadStmt alive #5315
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Related issue = #5284
The gradient accumulation to global field only happens when reading it, i.e., GlobalLoadStmt. However, the store forwarding kills some GlobalLoadStmt so that the accumulations for the grad of intermediate variable are skipped (as shown in #5284).
This PR makes the store forwarding skipped when dealing with GlobalLoadStmt who reads a global field with grad.