Disallow overriding initializers if IR version < 4 (#1324)
Description:
Disallow overriding an initializer via a graph input if the IR version is < 4. This enforces an implicit assumption that initializers should be treated as constant, and allows constant folding to be done on a model with an older IR version.
Separate constant and overridable initializers so that it's clear which ones constant folding can utilize.
Update Graph to not add all initializers to the graph inputs when the graph is manually created (i.e. not loaded from a GraphProto) and the IR version is >= 4.
Motivation and Context
In order to do constant folding we need to know which initializers can be treated as constant and which are overridable. All initializers were required to have a matching graph input prior to IR version 4, technically making all of them overridable. The intention however was for them to be treated as constants, and this change enforces that intent.
The benefit of doing so is that constant folding will work for models with IR version < 4. The cost is that if someone is actually overriding an initializer they will need to update the IR version of their model to version 4 in order to keep doing so. The belief is that this is a very small subset of usage (e.g. models involving feeding in a truncated sequence) and the cost to update that small subset is warranted by the benefit of constant folding being able to be enabled on all older models without them needing an IR version update.
