Until ERC 3156 is live on mainnet, here is my swiss army collection of templates to flash loan from each provider.

Aave V2 single asset Flash Loan
https://github.com/fifikobayashi/DeFi-Stuff/blob/master/AaveV2SingleAssetFlashDemo.sol

Aave V2 batch Flash Loan
https://github.com/fifikobayashi/AaveV2-BatchFlashDemo

Uniswap V2 Flash Loan (From Austin Williams)
https://github.com/Austin-Williams/uniswap-flash-swapper

Dydx Flash Loan
https://github.com/fifikobayashi/DeFi-Stuff/blob/master/DydxFlashLoanDemo.sol

Yield Flash Loan
https://github.com/fifikobayashi/YieldFlashDemo