fix pid_lagrange math

[AmineAndam04]

Description

The docstring for PIDLagrangian.pid_update states the update rule as:

$\lambda_{t+1} = \lambda_t + (K_p e_p + K_i \int e_p dt + K_d \frac{d e_p}{d t}) \eta$

However, this does not match the actual implementation: there is no learning rate $\eta$ and the update is not incremental ($\lambda_{t+1} = \lambda_t + \dots$).

I suggest to replace the docstring formula with the following:
$\lambda_{t+1} = \max(0,K_p e_p + K_i \int e_p dt + K_d \frac{d e_p}{d t})$

Motivation and Context

• I have raised an issue to propose this change ( issue [BUG] fix pid_lagrange math #391 )

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