[TimesFMv1] fix variance calculation

[kashif]

1. Masked variance calculation (lines 95-107): Changed from the numerically unstable E[X²] - E[X]² formula to the stable centered formula E[(X-μ)²]
2. Sigma clamping (line 609): Changed from torch.where(sigma < tolerance, 1.0, sigma) to torch.clamp(sigma, min=tolerance)

by: huggingface/transformers#42099 @AnMakc

[AnMakc]

@kashif
Thanks for bringing it here.
Could you also fix the jax version, it has the same issue:

timesfm/v1/src/timesfm/patched_decoder.py

Line 199 in bf88c5d

masked_var = masked_squared_sum / num_valid_elements - masked_mean**2

Btw, this potentially affected model pre-training as well.