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Fixes bug in SelectiveScanFn.forward for when B is not variable and last_state is returned #371

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Fixes bug in SelectiveScanFn.forward for when B is not variable and last_state is returned #371

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11 changes: 11 additions & 0 deletions mamba_ssm/ops/selective_scan_interface.py
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,9 @@ class SelectiveScanFn(torch.autograd.Function):
@staticmethod
def forward(ctx, u, delta, A, B, C, D=None, z=None, delta_bias=None, delta_softplus=False,
return_last_state=False):

is_variable_B = B.dim() >= 3

if u.stride(-1) != 1:
u = u.contiguous()
if delta.stride(-1) != 1:
Expand All @@ -43,6 +46,14 @@ def forward(ctx, u, delta, A, B, C, D=None, z=None, delta_bias=None, delta_softp
ctx.delta_softplus = delta_softplus
ctx.has_z = z is not None
last_state = x[:, :, -1, 1::2] # (batch, dim, dstate)

# 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.
if not is_variable_B:
last_state = torch.einsum('bdn,dn->bdn', last_state, B)

if not ctx.has_z:
ctx.save_for_backward(u, delta, A, B, C, D, delta_bias, x)
return out if not return_last_state else (out, last_state)
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