Indexing uses the pattern [start:stop:step].
In the first approach you will get all samples from the start index 127 to the end (so just the last sample), while the second approach will give you all samples from the beginning to index 1, excluding index 1 (so just the first sample):
x = torch.randn(128, 3, 64, 64)

a = x[127:]
b = x[:1]

print((a == x[-1]).all())
> tensor(True)
print((b == x[0]).all())
> tensor(True)