Unfortunately, the code is not executable. Could you create dummy inputs, so that we could debug this issue?
Using your default values (reduction='none', zero_infinity=True), this dummy code snippet works:
ctc = torch.nn.CTCLoss(reduction='none', zero_infinity=True)

T = 50      # Input sequence length
C = 20      # Number of classes (including blank)
N = 16      # Batch size
S = 30      # Target sequence length of longest target in batch
S_min = 10  # Minimum target length, for demonstration purposes
# Initialize random batch of input vectors, for *size = (T,N,C)
input = torch.randn(T, N, C).log_softmax(2).detach().requires_grad_()
# Initialize random batch of targets (0 = blank, 1:C = classes)
target = torch.randint(low=1, high=C, size=(N, S), dtype=torch.long)
input_lengths = torch.full(size=(N,), fill_value=T, dtype=torch.long)
target_lengths = torch.randint(low=S_min, high=S, size=(N,), dtype=torch.long)

loss = ctc(input, target, input_lengths, target_lengths)