De-templatize TensorContractionBase (Linalg example/tutorial)

    TensorContractionBase has become too unwieldy with all the CRTP manipulation once less trivial transformations are implemented.
    This CL drops CRTP for inheritance and uses the same name comparison trick to figure out what to cast into.
    As a byproduct, all the -inl.h files disappear.
    To maintain the separation between directories, a LINALG_STEP variable is introduced

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PiperOrigin-RevId: 242546977