What does librosa.load return?
Is it the instantaneous sound pressure in pa?


How would a piece of software know about sound pressure? What it gets is the digitized voltage from a microphone, with zero information about the physical sensitivity of that microphone.


  or the just the instantaneous amplitude of the sound signal with no unit?


Exactly! :)


  How can I use this information to calculate the SPL(no-weigting) of the signal?


Calibrate your recording setup with a sound source of known power. That's the only way short of buying an expensive calibrated measurement microphone, amplifier, digitizer. But then, there will be little need for librosa.