создавать вкладки и добавлять виджеты + кнопки в каждую вкладку с помощью цикла

Попытайся:

import sys
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtWidgets import QMainWindow, QWidget, QApplication

#from test import Ui_MainWindow
class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(447, 356)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.gridLayout_3 = QtWidgets.QGridLayout(self.centralwidget)
        self.gridLayout_3.setObjectName("gridLayout_3")
        self.tabWidget = QtWidgets.QTabWidget(self.centralwidget)
        self.tabWidget.setObjectName("tabWidget")
        self.tab = QtWidgets.QWidget()
        self.tab.setObjectName("tab")
        self.gridLayout_2 = QtWidgets.QGridLayout(self.tab)
        self.gridLayout_2.setObjectName("gridLayout_2")
        self.widget = QtWidgets.QWidget(self.tab)
        self.widget.setObjectName("widget")
        self.gridLayout_2.addWidget(self.widget, 1, 0, 1, 1)
        self.pushButton = QtWidgets.QPushButton(self.tab)
        self.pushButton.setObjectName("pushButton")
        self.gridLayout_2.addWidget(self.pushButton, 2, 0, 1, 1)
        self.tabWidget.addTab(self.tab, "")
        self.gridLayout_3.addWidget(self.tabWidget, 0, 0, 1, 1)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 447, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        self.tabWidget.setCurrentIndex(0)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.pushButton.setText(_translate("MainWindow", "PushButton"))
        self.tabWidget.setTabText(self.tabWidget.indexOf(self.tab), _translate("MainWindow", "Tab 1"))


#class Ui_MainWindow(QMainWindow, Ui_MainWindow):                              # ---
class MainWindow(QMainWindow, Ui_MainWindow):                                  # + MainWindow
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)                               # + MainWindow
       
        self.setupUi(self)
        self.pushButton.clicked.connect(lambda: print(self.pushButton.text())) # +
        
        self.list = ['a', 'b', 'c', 'd']
        self.populate()

    def populate(self):
#        for i in range(0, len(self.list)):
        for i in self.list:                                                     # +++
#            if i > 0:
            tab = QtWidgets.QWidget()                                           # - self.
            self.tabWidget.addTab(tab, i)                                       # i
#            self.tabWidget.setTabText(i, self.list[i])
        
            btn = QtWidgets.QPushButton(f'Button {i}')                          # - self.
            btn.clicked.connect(lambda ch, textBtn=btn.text(): print(textBtn))  # +++
            widget = QtWidgets.QWidget()                                        # - self. 
            widget.setStyleSheet(f"background: #{i}{i}{i};")                    # +
            
            grid = QtWidgets.QGridLayout(tab)                                   # - self. ; + (tab)
            grid.addWidget(widget, 0, 0)
            grid.addWidget(btn, 1, 0)


if __name__ == '__main__':
    app = QApplication(sys.argv)
#    prog = Ui_MainWindow()                                                     # ---
    prog = MainWindow()                                                         # + MainWindow
    prog.showMaximized()
    sys.exit(app.exec_())