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Income Taxes - Summary of reconciliation of the expected U.S. federal income tax rate to the effective tax rate (Detail) - USD ($)
3 Months Ended 6 Months Ended 12 Months Ended
Mar. 26, 2022
Mar. 27, 2021
Mar. 26, 2022
Mar. 27, 2021
Sep. 25, 2021
Sep. 26, 2020
Sep. 28, 2019
Sep. 28, 2019
Income Tax Disclosure [Abstract]                
Loss before income tax $ (29,903,000) $ (26,855,000) $ (52,956,000) $ (53,058,000) $ (122,314,000) $ (109,568,000) $ (104,361,000)  
Tax on pre-tax loss         (25,686,000)      
Loss not subject to tax         26,575,000      
Foreign rate differential         233,000      
Decrease in valuation allowance         (993,000)      
Other         (129,000)      
Total income tax $ 0 $ 0 $ 0 $ 0 $ 0 $ (47,000) $ 0 $ 0
Tax on pre-tax loss Percent         21.00%      
Loss not subject to tax Percent         (21.73%)      
Foreign rate differential Percent         (0.19%)      
Decrease in valuation allowance Percent         0.81%      
Other Percent         0.11%      
Total income tax Percent         0.00%