Add vecenv fallback and fix batched forward state#429

ry2009 wants to merge 2 commits intoPufferAI:4.0PufferAI/PufferLib:4.0from

UF-AppliedMLSystems-Spring24:sps-fallback-fixUF-AppliedMLSystems-Spring24/PufferLib:sps-fallback-fixCopy head branch name to clipboard