Fixes bug in SelectiveScanFn.forward for when B is not variable and last_state is returned #371
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…function has to return the last_state. # The cuda kernel does a peculiar optimization of not multiplying the state by B if B is not variable! This does not impact MambaInnerFn, because it never returns the state. But SelectiveScanFn may needd to return the last state! Hence the following is needed.
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test_selective_scan() fails when is_variable_B is False.
Turns out selective_scan_fwd_kernel incorporates an optimization of not multiplying the state by B if B is not variable. This does not impact MambaInnerFn, because MambaInnerFn never returns the state. But SelectiveScanFn may need to return the last_state. The changes to the code fix this problem, by multiplying the last_state by B before returning it when B is not variable.