Fix delayvar not correct in concat mode

[CloudyDory]

Description

The delay variable with "concat" update method seems not being updated correctly. See the following code for example:

import brainpy.math as bm

spike = bm.zeros(5, dtype=bm.bool)
delay_len = 2

print('Rotation mode:')
spike_buffer = bm.LengthDelay(spike, delay_len, update_method='rotation')
for i in range(5):
    spike[i] = True
    spike_buffer.update(spike)
    spike_delay = spike_buffer.retrieve(delay_len)
    
    print(spike.value)
    print(spike_delay)
    print('')
    
print('Concat mode:')
spike = bm.zeros(5, dtype=bm.bool)
spike_buffer = bm.LengthDelay(spike, delay_len=delay_len, update_method='concat')
for i in range(5):
    spike[i] = True
    spike_buffer.update(spike)
    spike_delay = spike_buffer.retrieve(delay_len)
    
    print(spike.value)
    print(spike_delay)
    print('')

Output:

Rotation mode:
[ True False False False False]
[False False False False False]

[ True  True False False False]
[False False False False False]

[ True  True  True False False]
[ True False False False False]

[ True  True  True  True False]
[ True  True False False False]

[ True  True  True  True  True]
[ True  True  True False False]

Concat mode:
[ True  True  True  True  True]
[False False False False False]

[ True  True  True  True  True]
[False False False False False]

[ True  True  True  True  True]
[False False False False False]

[ True  True  True  True  True]
[False False False False False]

[ True  True  True  True  True]
[False False False False False]

The cause is in line 476 of delayvars.py, the code only update the first row of the data buffer matrix.

After fixing the issue, the above test code can run correctly:

Rotation mode:
[ True False False False False]
[False False False False False]

[ True  True False False False]
[False False False False False]

[ True  True  True False False]
[ True False False False False]

[ True  True  True  True False]
[ True  True False False False]

[ True  True  True  True  True]
[ True  True  True False False]

Concat mode:
[ True False False False False]
[False False False False False]

[ True  True False False False]
[False False False False False]

[ True  True  True False False]
[ True False False False False]

[ True  True  True  True False]
[ True  True False False False]

[ True  True  True  True  True]
[ True  True  True False False]

[chaoming0625]

Great PR.