[SD3] Incorrect stochastic sampling implementation

### Describe the bug

Ref to Algorithm 2 of [EDM](https://arxiv.org/abs/2206.00364), for a given sample $x_t$, noise is introduced to it reaching a higher noise level $\hat{t}$, then we evaluate network with $\hat{x_t}$, $\hat{t}$ as input. However, the current implementation evaluates network with $x_t$, $t$ as input, which is inconsistent from definition.

Current implementation is more similar with Euler-Maruyama in spirit, "One can interpret Euler–Maruyama as first adding
noise and then performing an ODE step, not from the intermediate state after noise injection, but
assuming that $x$ and $\sigma$ remained at the initial state at the beginning of the iteration step." quote from [EDM](https://arxiv.org/abs/2206.00364)

### Reproduction

no

### Logs

_No response_

### System Info

no

### Who can help?

@yiyixuxu @sayakpaul

No file, test, or entry point is named. Start by locating the stochastic sampling implementation and compare its network inputs with Algorithm 2 of the linked EDM paper, focusing on whether the noise-adjusted sample and noise level are evaluated. Done means the implementation follows that algorithm and has coverage demonstrating the corrected inputs.

Scritto dal modello di indicizzazione a partire dal testo della issue.